diff --git a/src/matrix/e2ee/olm/Decryption.js b/src/matrix/e2ee/olm/Decryption.js
index f701f4df..ec87af74 100644
--- a/src/matrix/e2ee/olm/Decryption.js
+++ b/src/matrix/e2ee/olm/Decryption.js
@@ -15,6 +15,7 @@ limitations under the License.
 */
 
 import {DecryptionError} from "../common.js";
+import {groupBy} from "../../../utils/groupBy.js";
 
 const SESSION_LIMIT_PER_SENDER_KEY = 4;
 
@@ -49,16 +50,7 @@ export class Decryption {
     // 
     // doing it one by one would be possible, but we would lose the opportunity for parallelization
     async decryptAll(events) {
-        const eventsPerSenderKey = events.reduce((map, event) => {
-            const senderKey = event.content?.["sender_key"];
-            let list = map.get(senderKey);
-            if (!list) {
-                list = [];
-                map.set(senderKey, list);
-            }
-            list.push(event);
-            return map;
-        }, new Map());
+        const eventsPerSenderKey = groupBy(events, event => event.content?.["sender_key"]);
         const timestamp = this._now();
         const readSessionsTxn = await this._storage.readTxn([this._storage.storeNames.olmSessions]);
         // decrypt events for different sender keys in parallel
diff --git a/src/utils/groupBy.js b/src/utils/groupBy.js
new file mode 100644
index 00000000..5df2f36d
--- /dev/null
+++ b/src/utils/groupBy.js
@@ -0,0 +1,35 @@
+/*
+Copyright 2020 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.
+*/
+
+export function groupBy(array, groupFn) {
+    return groupByWithCreator(array, groupFn,
+        () => {return [];},
+        (array, value) => array.push(value)
+    );
+}
+
+export function groupByWithCreator(array, groupFn, createCollectionFn, addCollectionFn) {
+    return array.reduce((map, value) => {
+        const key = groupFn(value);
+        let collection = map.get(key);
+        if (!collection) {
+            collection = createCollectionFn();
+            map.set(key, collection);
+        }
+        addCollectionFn(collection, value);
+        return map;
+    }, new Map());
+}