Throttle RoomState.members handler to improve performance

Lazy Loading emits a RoomState.members for each member which in the case of a room like
Matrix HQ means it happens over 8000 times in a very short span of time causing the UI
to lock up.

src/components/views/right_panel/RoomSummaryCard.tsx

@@ -43,6 +43,7 @@ import RoomContext from "../../../contexts/RoomContext";
 import {UIFeature} from "../../../settings/UIFeature";
 import {ChevronFace, ContextMenuTooltipButton, useContextMenu} from "../../structures/ContextMenu";
 import WidgetContextMenu from "../context_menus/WidgetContextMenu";
+import {useRoomMembers} from "../../../hooks/useRoomMembers";
 
 interface IProps {
     room: Room;
@@ -210,14 +211,6 @@ const onRoomSettingsClick = () => {
     defaultDispatcher.dispatch({ action: "open_room_settings" });
 };
 
-const useMemberCount = (room: Room) => {
-    const [count, setCount] = useState(room.getJoinedMembers().length);
-    useEventEmitter(room.currentState, "RoomState.members", () => {
-        setCount(room.getJoinedMembers().length);
-    });
-    return count;
-};
-
 const RoomSummaryCard: React.FC<IProps> = ({ room, onClose }) => {
     const cli = useContext(MatrixClientContext);
 
@@ -251,12 +244,12 @@ const RoomSummaryCard: React.FC<IProps> = ({ room, onClose }) => {
         </div>
     </React.Fragment>;
 
-    const memberCount = useMemberCount(room);
+    const members = useRoomMembers(room);
 
     return <BaseCard header={header} className="mx_RoomSummaryCard" onClose={onClose}>
         <Group title={_t("About")} className="mx_RoomSummaryCard_aboutGroup">
             <Button className="mx_RoomSummaryCard_icon_people" onClick={onRoomMembersClick}>
-                {_t("%(count)s people", { count: memberCount })}
+                {_t("%(count)s people", { count: members.length })}
             </Button>
             <Button className="mx_RoomSummaryCard_icon_files" onClick={onRoomFilesClick}>
                 {_t("Show files")}

src/hooks/useRoomMembers.ts

@@ -0,0 +1,31 @@
+/*
+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.
+*/
+
+import {useState} from "react";
+import {Room} from "matrix-js-sdk/src/models/room";
+import {RoomMember} from "matrix-js-sdk/src/models/room-member";
+
+import {useEventEmitter} from "./useEventEmitter";
+import {throttle} from "lodash";
+
+// Hook to simplify listening to Matrix Room members
+export const useRoomMembers = (room: Room, throttleWait = 250) => {
+    const [members, setMembers] = useState<RoomMember[]>(room.getJoinedMembers());
+    useEventEmitter(room.currentState, "RoomState.members", throttle(() => {
+        setMembers(room.getJoinedMembers());
+    }, throttleWait, {leading: true, trailing: true}));
+    return members;
+};