diff --git a/src/matrix/Sync.js b/src/matrix/Sync.js
index 4198618a..1295e7db 100644
--- a/src/matrix/Sync.js
+++ b/src/matrix/Sync.js
@@ -87,12 +87,16 @@ export class Sync {
     }
 
     async _syncLoop(syncToken) {
+        let afterSyncCompletedPromise = Promise.resolve();
         // if syncToken is falsy, it will first do an initial sync ... 
         while(this._status.get() !== SyncStatus.Stopped) {
+            let roomChanges;
             try {
                 console.log(`starting sync request with since ${syncToken} ...`);
                 const timeout = syncToken ? INCREMENTAL_TIMEOUT : undefined; 
-                syncToken = await this._syncRequest(syncToken, timeout);
+                const syncResult = await this._syncRequest(syncToken, timeout, afterSyncCompletedPromise);
+                syncToken = syncResult.syncToken;
+                roomChanges = syncResult.roomChanges;
                 this._status.set(SyncStatus.Syncing);
             } catch (err) {
                 if (!(err instanceof AbortError)) {
@@ -101,18 +105,39 @@ export class Sync {
                 }
             }
             if (!this._error) {
-                try {
-                    // TODO: run this in parallel with the next sync request
-                    await this._session.afterSyncCompleted();
-                } catch (err) {
-                    console.error("error during after sync completed, continuing to sync.",  err.stack);
-                    // swallowing error here apart from logging
-                }
+                afterSyncCompletedPromise = this._runAfterSyncCompleted(roomChanges);
             }
         }
     }
 
-    async _syncRequest(syncToken, timeout) {
+    async _runAfterSyncCompleted(roomChanges) {
+        const sessionPromise = (async () => {
+            try {
+                await this._session.afterSyncCompleted();
+            } catch (err) {
+                console.error("error during session afterSyncCompleted, continuing",  err.stack);
+            }
+        })();
+        let allPromises = [sessionPromise];
+
+        const roomsNeedingAfterSyncCompleted = roomChanges.filter(rc => {
+            return rc.changes.needsAfterSyncCompleted;
+        });
+        if (roomsNeedingAfterSyncCompleted.length) {
+            allPromises = allPromises.concat(roomsNeedingAfterSyncCompleted.map(async ({room, changes}) => {
+                try {
+                    await room.afterSyncCompleted(changes);
+                } catch (err) {
+                    console.error(`error during room ${room.id} afterSyncCompleted, continuing`,  err.stack);
+                }
+            }));
+        }
+        // run everything in parallel,
+        // we don't want to delay the next sync too much
+        await Promise.all(allPromises);
+    }
+
+    async _syncRequest(syncToken, timeout, prevAfterSyncCompletedPromise) {
         let {syncFilterId} = this._session;
         if (typeof syncFilterId !== "string") {
             this._currentRequest = this._hsApi.createFilter(this._session.user.id, {room: {state: {lazy_load_members: true}}});
@@ -121,43 +146,20 @@ export class Sync {
         const totalRequestTimeout = timeout + (80 * 1000);  // same as riot-web, don't get stuck on wedged long requests
         this._currentRequest = this._hsApi.sync(syncToken, syncFilterId, timeout, {timeout: totalRequestTimeout});
         const response = await this._currentRequest.response();
+        // wait here for the afterSyncCompleted step of the previous sync to complete
+        // before we continue processing this sync response
+        await prevAfterSyncCompletedPromise;
+
         const isInitialSync = !syncToken;
         syncToken = response.next_batch;
-        const storeNames = this._storage.storeNames;
-        const syncTxn = await this._storage.readWriteTxn([
-            storeNames.session,
-            storeNames.roomSummary,
-            storeNames.roomState,
-            storeNames.roomMembers,
-            storeNames.timelineEvents,
-            storeNames.timelineFragments,
-            storeNames.pendingEvents,
-            storeNames.userIdentities,
-            storeNames.inboundGroupSessions,
-            storeNames.groupSessionDecryptions,
-            storeNames.deviceIdentities,
-        ]);
-        const roomChanges = [];
+        const syncTxn = await this._openSyncTxn();
+        let roomChanges = [];
         let sessionChanges;
         try {
             // to_device
             // presence
             if (response.rooms) {
-                const promises = parseRooms(response.rooms, async (roomId, roomResponse, membership) => {
-                    // ignore rooms with empty timelines during initial sync,
-                    // see https://github.com/vector-im/hydrogen-web/issues/15
-                    if (isInitialSync && timelineIsEmpty(roomResponse)) {
-                        return;
-                    }
-                    let room = this._session.rooms.get(roomId);
-                    if (!room) {
-                        room = this._session.createRoom(roomId);
-                    }
-                    console.log(` * applying sync response to room ${roomId} ...`);
-                    const changes = await room.writeSync(roomResponse, membership, isInitialSync, syncTxn);
-                    roomChanges.push({room, changes});
-                });
-                await Promise.all(promises);
+                roomChanges = await this._writeRoomResponses(response.rooms, isInitialSync, syncTxn);
             }
             sessionChanges = await this._session.writeSync(response, syncFilterId, roomChanges, syncTxn);
         } catch(err) {
@@ -182,7 +184,44 @@ export class Sync {
             room.afterSync(changes);
         }
 
-        return syncToken;
+        return {syncToken, roomChanges};
+    }
+
+    async _writeRoomResponses(roomResponses, isInitialSync, syncTxn) {
+        const roomChanges = [];
+        const promises = parseRooms(roomResponses, async (roomId, roomResponse, membership) => {
+            // ignore rooms with empty timelines during initial sync,
+            // see https://github.com/vector-im/hydrogen-web/issues/15
+            if (isInitialSync && timelineIsEmpty(roomResponse)) {
+                return;
+            }
+            let room = this._session.rooms.get(roomId);
+            if (!room) {
+                room = this._session.createRoom(roomId);
+            }
+            console.log(` * applying sync response to room ${roomId} ...`);
+            const changes = await room.writeSync(roomResponse, membership, isInitialSync, syncTxn);
+            roomChanges.push({room, changes});
+        });
+        await Promise.all(promises);
+        return roomChanges;
+    }
+
+    async _openSyncTxn() {
+        const storeNames = this._storage.storeNames;
+        return await this._storage.readWriteTxn([
+            storeNames.session,
+            storeNames.roomSummary,
+            storeNames.roomState,
+            storeNames.roomMembers,
+            storeNames.timelineEvents,
+            storeNames.timelineFragments,
+            storeNames.pendingEvents,
+            storeNames.userIdentities,
+            storeNames.inboundGroupSessions,
+            storeNames.groupSessionDecryptions,
+            storeNames.deviceIdentities,
+        ]);
     }
 
     stop() {