Prioritise and reduce the amount of events decrypted on application startup (#5980)

src/components/structures/MatrixChat.tsx

@@ -908,6 +908,11 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
             let presentedId = roomInfo.room_alias || roomInfo.room_id;
             const room = MatrixClientPeg.get().getRoom(roomInfo.room_id);
             if (room) {
+                // Not all timeline events are decrypted ahead of time anymore
+                // Only the critical ones for a typical UI are
+                // This will start the decryption process for all events when a
+                // user views a room
+                room.decryptAllEvents();
                 const theAlias = Rooms.getDisplayAliasForRoom(room);
                 if (theAlias) {
                     presentedId = theAlias;

src/components/structures/TimelinePanel.js

@@ -38,6 +38,7 @@ import {haveTileForEvent} from "../views/rooms/EventTile";
 import {UIFeature} from "../../settings/UIFeature";
 import {objectHasDiff} from "../../utils/objects";
 import {replaceableComponent} from "../../utils/replaceableComponent";
+import { arrayFastClone } from "../../utils/arrays";
 
 const PAGINATE_SIZE = 20;
 const INITIAL_SIZE = 20;
@@ -1141,6 +1142,18 @@ class TimelinePanel extends React.Component {
     // get the list of events from the timeline window and the pending event list
     _getEvents() {
         const events = this._timelineWindow.getEvents();
+
+        // `arrayFastClone` performs a shallow copy of the array
+        // we want the last event to be decrypted first but displayed last
+        // `reverse` is destructive and unfortunately mutates the "events" array
+        arrayFastClone(events)
+            .reverse()
+            .forEach(event => {
+                if (event.shouldAttemptDecryption()) {
+                    event.attemptDecryption(MatrixClientPeg.get()._crypto);
+                }
+            });
+
         const firstVisibleEventIndex = this._checkForPreJoinUISI(events);
 
         // Hold onto the live events separately. The read receipt and read marker