diff --git a/scripts/babel-test.js b/scripts/babel-test.js
new file mode 100644
index 00000000..24283dc3
--- /dev/null
+++ b/scripts/babel-test.js
@@ -0,0 +1,32 @@
+babel = require('@babel/standalone');
+
+const code = `
+async function doit() {
+    const foo = {bar: 5};
+    const mapped = Object.values(foo).map(n => n*n);
+    console.log(mapped);
+    await Promise.resolve();
+}
+doit();
+`;
+
+const {code: babelCode} = babel.transform(code, {
+    babelrc: false,
+    configFile: false,
+    presets: [
+        [
+            "env",
+            {
+                useBuiltIns: "entry",
+                modules: false,
+                corejs: "3.4",
+                targets: "IE 11",
+                // we provide our own promise polyfill (es6-promise)
+                // with support for synchronous flushing of
+                // the queue for idb where needed 
+                // exclude: ["es.promise", "es.promise.all-settled", "es.promise.finally"]
+            }
+        ]
+    ]
+});
+console.log(babelCode);
diff --git a/src/matrix/e2ee/megolm/Decryption.js b/src/matrix/e2ee/megolm/Decryption.js
index 8f4714ea..30e1480f 100644
--- a/src/matrix/e2ee/megolm/Decryption.js
+++ b/src/matrix/e2ee/megolm/Decryption.js
@@ -118,6 +118,7 @@ export class Decryption {
         // look only in the cache after looking into newKeys as it may contains that are better
         if (!sessionInfo) {
             sessionInfo = sessionCache.get(roomId, senderKey, sessionId);
+            // TODO: shouldn't we retain here?
         }
         if (!sessionInfo) {
             const sessionEntry = await txn.inboundGroupSessions.get(roomId, senderKey, sessionId);
diff --git a/src/matrix/e2ee/megolm/decryption/ISessionSource.ts b/src/matrix/e2ee/megolm/decryption/ISessionSource.ts
new file mode 100644
index 00000000..8a96ba12
--- /dev/null
+++ b/src/matrix/e2ee/megolm/decryption/ISessionSource.ts
@@ -0,0 +1,26 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+//import type {InboundGroupSession} from "@matrix-org/olm";
+interface InboundGroupSession {}
+
+export interface ISessionSource {
+	deserializeIntoSession(session: InboundGroupSession): void;
+	get roomId(): string;
+    get senderKey(): string;
+    get sessionId(): string;
+    get claimedEd25519Key(): string;
+}
diff --git a/src/matrix/e2ee/megolm/decryption/RoomKey.js b/src/matrix/e2ee/megolm/decryption/RoomKey.js
deleted file mode 100644
index 6fcd738b..00000000
--- a/src/matrix/e2ee/megolm/decryption/RoomKey.js
+++ /dev/null
@@ -1,166 +0,0 @@
-import {SessionInfo} from "./SessionInfo.js";
-
-export class BaseRoomKey {
-    constructor() {
-        this._sessionInfo = null;
-        this._isBetter = null;
-        this._eventIds = null;
-    }
-
-    async createSessionInfo(olm, pickleKey, txn) {
-        if (this._isBetter === false) {
-            return;
-        }
-        const session = new olm.InboundGroupSession();
-        try {
-            this._loadSessionKey(session);
-            this._isBetter = await this._isBetterThanKnown(session, olm, pickleKey, txn);
-            if (this._isBetter) {
-                const claimedKeys = {ed25519: this.claimedEd25519Key};
-                this._sessionInfo = new SessionInfo(this.roomId, this.senderKey, session, claimedKeys);
-                // retain the session so we don't have to create a new session during write.
-                this._sessionInfo.retain();
-                return this._sessionInfo;
-            } else {
-                session.free();
-                return;
-            }
-        } catch (err) {
-            this._sessionInfo = null;
-            session.free();
-            throw err;
-        }
-    }
-
-    async _isBetterThanKnown(session, olm, pickleKey, txn) {
-        let isBetter = true;
-        // TODO: we could potentially have a small speedup here if we looked first in the SessionCache here...
-        const existingSessionEntry = await txn.inboundGroupSessions.get(this.roomId, this.senderKey, this.sessionId);
-        if (existingSessionEntry?.session) {
-            const existingSession = new olm.InboundGroupSession();
-            try {
-                existingSession.unpickle(pickleKey, existingSessionEntry.session);
-                isBetter = session.first_known_index() < existingSession.first_known_index();
-            } finally {
-                existingSession.free();
-            }
-        }
-        // store the event ids that can be decrypted with this key
-        // before we overwrite them if called from `write`.
-        if (existingSessionEntry?.eventIds) {
-            this._eventIds = existingSessionEntry.eventIds;
-        }
-        return isBetter;
-    }
-
-    async write(olm, pickleKey, txn) {
-        // we checked already and we had a better session in storage, so don't write
-        if (this._isBetter === false) {
-            return false;
-        }
-        if (!this._sessionInfo) {
-            await this.createSessionInfo(olm, pickleKey, txn);
-        }
-        if (this._sessionInfo) {
-            const session = this._sessionInfo.session;
-            const sessionEntry = {
-                roomId: this.roomId,
-                senderKey: this.senderKey,
-                sessionId: this.sessionId,
-                session: session.pickle(pickleKey),
-                claimedKeys: this._sessionInfo.claimedKeys,
-            };
-            txn.inboundGroupSessions.set(sessionEntry);
-            this.dispose();
-            return true;
-        }
-        return false;
-    }
-
-    get eventIds() {
-        return this._eventIds;
-    }
-
-    dispose() {
-        if (this._sessionInfo) {
-            this._sessionInfo.release();
-            this._sessionInfo = null;
-        }
-    }
-}
-
-class DeviceMessageRoomKey extends BaseRoomKey {
-    constructor(decryptionResult) {
-        super();
-        this._decryptionResult = decryptionResult;
-    }
-
-    get roomId() { return this._decryptionResult.event.content?.["room_id"]; }
-    get senderKey() { return this._decryptionResult.senderCurve25519Key; }
-    get sessionId() { return this._decryptionResult.event.content?.["session_id"]; }
-    get claimedEd25519Key() { return this._decryptionResult.claimedEd25519Key; }
-
-    _loadSessionKey(session) {
-        const sessionKey = this._decryptionResult.event.content?.["session_key"];
-        session.create(sessionKey);
-    }
-}
-
-class BackupRoomKey extends BaseRoomKey {
-    constructor(roomId, sessionId, backupInfo) {
-        super();
-        this._roomId = roomId;
-        this._sessionId = sessionId;
-        this._backupInfo = backupInfo;
-    }
-
-    get roomId() { return this._roomId; }
-    get senderKey() { return this._backupInfo["sender_key"]; }
-    get sessionId() { return this._sessionId; }
-    get claimedEd25519Key() { return this._backupInfo["sender_claimed_keys"]?.["ed25519"]; }
-
-    _loadSessionKey(session) {
-        const sessionKey = this._backupInfo["session_key"];
-        session.import_session(sessionKey);
-    }
-}
-
-export function fromDeviceMessage(dr) {
-    const roomId = dr.event.content?.["room_id"];
-    const sessionId = dr.event.content?.["session_id"];
-    const sessionKey = dr.event.content?.["session_key"];
-    if (
-        typeof roomId === "string" || 
-        typeof sessionId === "string" || 
-        typeof senderKey === "string" ||
-        typeof sessionKey === "string"
-    ) {
-        return new DeviceMessageRoomKey(dr);
-    }
-}
-
-/*
-sessionInfo is a response from key backup and has the following keys:
-    algorithm
-    forwarding_curve25519_key_chain
-    sender_claimed_keys
-    sender_key
-    session_key
- */
-export function fromBackup(roomId, sessionId, sessionInfo) {
-    const sessionKey = sessionInfo["session_key"];
-    const senderKey = sessionInfo["sender_key"];
-    // TODO: can we just trust this?
-    const claimedEd25519Key = sessionInfo["sender_claimed_keys"]?.["ed25519"];
-
-    if (
-        typeof roomId === "string" && 
-        typeof sessionId === "string" && 
-        typeof senderKey === "string" &&
-        typeof sessionKey === "string" &&
-        typeof claimedEd25519Key === "string"
-    ) {
-        return new BackupRoomKey(roomId, sessionId, sessionInfo);
-    }
-}
-
diff --git a/src/matrix/e2ee/megolm/decryption/RoomKey.ts b/src/matrix/e2ee/megolm/decryption/RoomKey.ts
new file mode 100644
index 00000000..8815a4e6
--- /dev/null
+++ b/src/matrix/e2ee/megolm/decryption/RoomKey.ts
@@ -0,0 +1,300 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import type {InboundGroupSession} from "../../../storage/idb/stores/InboundGroupSessionStore";
+import type {Transaction} from "../../../storage/idb/Transaction";
+import type {DecryptionResult} from "../../DecryptionResult";
+
+declare class OlmInboundGroupSession {
+    constructor();
+    free(): void;
+    pickle(key: string | Uint8Array): string;
+    unpickle(key: string | Uint8Array, pickle: string);
+    create(session_key: string): string;
+    import_session(session_key: string): string;
+    decrypt(message: string): object;
+    session_id(): string;
+    first_known_index(): number;
+    export_session(message_index: number): string;
+}
+
+export interface IRoomKey {
+    get roomId(): string;
+    get senderKey(): string;
+    get sessionId(): string;
+    get claimedEd25519Key(): string;
+    get eventIds(): string[] | undefined;
+    deserializeInto(session: OlmInboundGroupSession, pickleKey: string): void;
+}
+
+export interface IIncomingRoomKey extends IRoomKey {
+    copyEventIds(value: string[]): void;
+}
+
+export async function checkBetterKeyInStorage(key: IIncomingRoomKey, keyDeserialization: KeyDeserialization, txn: Transaction) {
+    let existingKey = keyDeserialization.cache.get(key.roomId, key.senderKey, key.sessionId);
+    if (!existingKey) {
+        const storageKey = await fromStorage(key.roomId, key.senderKey, key.sessionId, txn);
+        // store the event ids that can be decrypted with this key
+        // before we overwrite them if called from `write`.
+        if (storageKey) {
+            if (storageKey.eventIds) {
+                key.copyEventIds(storageKey.eventIds);
+            }
+            if (storageKey.hasSession) {
+                existingKey = storageKey;
+            }
+        }
+    }
+    if (existingKey) {
+        const isBetter = await keyDeserialization.useKey(key, newSession => {
+            return keyDeserialization.useKey(existingKey, existingSession => {
+                return newSession.first_known_index() < existingSession.first_known_index();
+            });
+        });
+        return isBetter ? key : existingKey;
+    } else {
+        return key;
+    }
+}
+
+async function write(olm, pickleKey, keyDeserialization, txn) {
+    // we checked already and we had a better session in storage, so don't write
+    if (this._isBetter === false) {
+        return false;
+    }
+    if (!this._sessionInfo) {
+        await this.createSessionInfo(olm, pickleKey, txn);
+    }
+    if (this._sessionInfo) {
+        // before calling write in parallel, we need to check keyDeserialization.running is false so we are sure our transaction will not be closed
+        const pickledSession = await keyDeserialization.useKey(this, session => session.pickle(pickleKey));
+        const sessionEntry = {
+            roomId: this.roomId,
+            senderKey: this.senderKey,
+            sessionId: this.sessionId,
+            session: pickledSession,
+            claimedKeys: this._sessionInfo.claimedKeys,
+        };
+        txn.inboundGroupSessions.set(sessionEntry);
+        this.dispose();
+        return true;
+    }
+    return false;
+}
+
+class BaseIncomingRoomKey {
+    private _eventIds?: string[];
+
+    get eventIds() { return this._eventIds; }
+
+    copyEventIds(eventIds: string[]): void {
+        this._eventIds = eventIds;
+    }
+}
+
+class DeviceMessageRoomKey extends BaseIncomingRoomKey implements IIncomingRoomKey {
+    private _decryptionResult: DecryptionResult;
+
+    constructor(decryptionResult: DecryptionResult) {
+        super();
+        this._decryptionResult = decryptionResult;
+    }
+
+    get roomId() { return this._decryptionResult.event.content?.["room_id"]; }
+    get senderKey() { return this._decryptionResult.senderCurve25519Key; }
+    get sessionId() { return this._decryptionResult.event.content?.["session_id"]; }
+    get claimedEd25519Key() { return this._decryptionResult.claimedEd25519Key; }
+
+    deserializeInto(session) {
+        const sessionKey = this._decryptionResult.event.content?.["session_key"];
+        session.create(sessionKey);
+    }
+}
+
+class BackupRoomKey extends BaseIncomingRoomKey implements IIncomingRoomKey {
+    private _roomId: string;
+    private _sessionId: string;
+    private _backupInfo: string;
+
+    constructor(roomId, sessionId, backupInfo) {
+        super();
+        this._roomId = roomId;
+        this._sessionId = sessionId;
+        this._backupInfo = backupInfo;
+    }
+
+    get roomId() { return this._roomId; }
+    get senderKey() { return this._backupInfo["sender_key"]; }
+    get sessionId() { return this._sessionId; }
+    get claimedEd25519Key() { return this._backupInfo["sender_claimed_keys"]?.["ed25519"]; }
+
+    deserializeInto(session) {
+        const sessionKey = this._backupInfo["session_key"];
+        session.import_session(sessionKey);
+    }
+}
+
+class StoredRoomKey implements IRoomKey {
+    private storageEntry: InboundGroupSession;
+
+    constructor(storageEntry: InboundGroupSession) {
+        this.storageEntry = storageEntry;
+    }
+
+    get roomId() { return this.storageEntry.roomId; }
+    get senderKey() { return this.storageEntry.senderKey; }
+    get sessionId() { return this.storageEntry.sessionId; }
+    get claimedEd25519Key() { return this.storageEntry.claimedKeys!["ed25519"]; }
+    get eventIds() { return this.storageEntry.eventIds; }
+
+    deserializeInto(session, pickleKey) {
+        session.unpickle(pickleKey, this.storageEntry.session);
+    }
+
+    get hasSession() {
+        // sessions are stored before they are received
+        // to keep track of events that need it to be decrypted.
+        // This is used to retry decryption of those events once the session is received.
+        return !!this.storageEntry.session;
+    }
+}
+
+export function fromDeviceMessage(dr) {
+    const roomId = dr.event.content?.["room_id"];
+    const sessionId = dr.event.content?.["session_id"];
+    const sessionKey = dr.event.content?.["session_key"];
+    if (
+        typeof roomId === "string" || 
+        typeof sessionId === "string" || 
+        typeof senderKey === "string" ||
+        typeof sessionKey === "string"
+    ) {
+        return new DeviceMessageRoomKey(dr);
+    }
+}
+
+/*
+sessionInfo is a response from key backup and has the following keys:
+    algorithm
+    forwarding_curve25519_key_chain
+    sender_claimed_keys
+    sender_key
+    session_key
+ */
+export function fromBackup(roomId, sessionId, sessionInfo) {
+    const sessionKey = sessionInfo["session_key"];
+    const senderKey = sessionInfo["sender_key"];
+    // TODO: can we just trust this?
+    const claimedEd25519Key = sessionInfo["sender_claimed_keys"]?.["ed25519"];
+
+    if (
+        typeof roomId === "string" && 
+        typeof sessionId === "string" && 
+        typeof senderKey === "string" &&
+        typeof sessionKey === "string" &&
+        typeof claimedEd25519Key === "string"
+    ) {
+        return new BackupRoomKey(roomId, sessionId, sessionInfo);
+    }
+}
+
+export async function fromStorage(roomId: string, senderKey: string, sessionId: string, txn: Transaction): Promise<StoredRoomKey | undefined> {
+    const existingSessionEntry = await txn.inboundGroupSessions.get(roomId, senderKey, sessionId);
+    if (existingSessionEntry) {
+        return new StoredRoomKey(existingSessionEntry);
+    }
+    return;
+}
+/*
+Because Olm only has very limited memory available when compiled to wasm,
+we limit the amount of sessions held in memory.
+*/
+class KeyDeserialization {
+
+    public readonly cache: SessionCache;
+    private pickleKey: string;
+    private olm: any;
+    private resolveUnusedEntry?: () => void;
+    private entryBecomesUnusedPromise?: Promise<void>;
+
+    constructor({olm, pickleKey, limit}) {
+        this.cache = new SessionCache(limit);
+        this.pickleKey = pickleKey;
+        this.olm = olm;
+    }
+
+    async useKey<T>(key: IRoomKey, callback: (session: OlmInboundGroupSession) => Promise<T> | T): Promise<T> {
+        const cacheEntry = await this.allocateEntry(key);
+        try {
+            const {session} = cacheEntry;
+            key.deserializeInto(session, this.pickleKey);
+            return await callback(session);
+        } finally {
+            this.freeEntry(cacheEntry);
+        }
+    }
+
+    get running() {
+        return !!this.cache.find(entry => entry.inUse);
+    }
+
+    private async allocateEntry(key): CacheEntry {
+        let entry;
+        if (this.cache.size >= MAX) {
+            while(!(entry = this.cache.find(entry => !entry.inUse))) {
+                await this.entryBecomesUnused();
+            }
+            entry.inUse = true;
+            entry.key = key;
+        } else {
+            const session: OlmInboundGroupSession = new this.olm.InboundGroupSession();
+            const entry = new CacheEntry(key, session);
+            this.cache.add(entry);
+        }
+        return entry;
+    }
+
+    private freeEntry(entry) {
+        entry.inUse = false;
+        if (this.resolveUnusedEntry) {
+            this.resolveUnusedEntry();
+            // promise is resolved now, we'll need a new one for next await so clear
+            this.entryBecomesUnusedPromise = this.resolveUnusedEntry = undefined;
+        }
+    }
+
+    private entryBecomesUnused(): Promise<void> {
+        if (!this.entryBecomesUnusedPromise) {
+            this.entryBecomesUnusedPromise = new Promise(resolve => {
+                this.resolveUnusedEntry = resolve;
+            });
+        }
+        return this.entryBecomesUnusedPromise;
+    }
+}
+
+class CacheEntry {
+    inUse: boolean;
+    session: OlmInboundGroupSession;
+    key: IRoomKey;
+
+    constructor(key, session) {
+        this.key = key;
+        this.session = session;
+        this.inUse = true;
+    }
+}
diff --git a/src/matrix/e2ee/megolm/decryption/SessionCache.js b/src/matrix/e2ee/megolm/decryption/SessionCache.js
index c5b2c0fb..712e2e50 100644
--- a/src/matrix/e2ee/megolm/decryption/SessionCache.js
+++ b/src/matrix/e2ee/megolm/decryption/SessionCache.js
@@ -33,11 +33,13 @@ export class SessionCache extends BaseLRUCache {
      * @return {SessionInfo?}
      */
     get(roomId, senderKey, sessionId) {
-        return this._get(s => {
+        const sessionInfo = this._get(s => {
             return s.roomId === roomId &&
                 s.senderKey === senderKey &&
                 sessionId === s.sessionId;
         });
+        sessionInfo?.retain();
+        return sessionInfo;
     }
 
     add(sessionInfo) {
diff --git a/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts b/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts
index 5dc0205f..65ce99ce 100644
--- a/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts
+++ b/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts
@@ -17,7 +17,7 @@ limitations under the License.
 import {MIN_UNICODE, MAX_UNICODE} from "./common";
 import {Store} from "../Store";
 
-interface InboundGroupSession {
+export interface InboundGroupSession {
     roomId: string;
     senderKey: string;
     sessionId: string;
diff --git a/src/utils/LRUCache.js b/src/utils/LRUCache.js
index 185e5aeb..183fc995 100644
--- a/src/utils/LRUCache.js
+++ b/src/utils/LRUCache.js
@@ -54,6 +54,16 @@ export class BaseLRUCache {
         }
     }
 
+    find(callback) {
+        // iterate backwards so least recently used items are found first
+        for (let i = this._entries.length - 1; i >= 0; i -= 1) {
+            const entry = this._entries[i];
+            if (callback(entry)) {
+                return entry;
+            }
+        }
+    }
+
     _onEvictEntry() {}
 }