fix: high cpu usage with large amount of channels

.changeset/quiet-cars-smile.md

@@ -0,0 +1,5 @@
+---
+"@rocket.chat/meteor": patch
+---
+
+Fixes client slowdown for users with large amount of channels

apps/meteor/client/views/root/hooks/loggedIn/useUnread.ts

@@ -1,13 +1,15 @@
 import { manageFavicon } from '@rocket.chat/favicon';
 import { useSession, useSessionDispatch, useUserPreference, useUserSubscriptions } from '@rocket.chat/ui-contexts';
-import { useEffect } from 'react';
+import { useEffect, useRef } from 'react';
 
 import { useFireGlobalEvent } from '../../../../hooks/useFireGlobalEvent';
 
 const query = { open: { $ne: false }, hideUnreadStatus: { $ne: true }, archived: { $ne: true } };
 const options = { fields: { unread: 1, alert: 1, rid: 1, t: 1, name: 1, ls: 1, unreadAlert: 1, fname: 1, prid: 1 } };
 const updateFavicon = manageFavicon();
 
+type UnreadData = { unread: number; alert: boolean | undefined; unreadAlert: string | undefined };
+
 export const useUnread = () => {
 	const unreadAlertEnabled = useUserPreference('unreadAlert');
 	const setUnread = useSessionDispatch('unread');
@@ -18,37 +20,48 @@ export const useUnread = () => {
 
 	const subscriptions = useUserSubscriptions(query, options);
 
+	// We keep a lightweight snapshot of the last emitted per-subscription unread state so we only
+	// fire "unread-changed-by-subscription" for subscriptions whose unread-relevant fields changed.
+	// Previously we emitted one global event per subscription on ANY change, which scaled O(N)
+	// with the user subscription count (thousands) for every single message event, dominating CPU.
+	const prevSubsRef = useRef(new Map<string, UnreadData>());
+
 	useEffect(() => {
-		let unreadAlert: false | '•' = false;
+		let badgeIndicator: false | '•' = false;
+		let unreadCount = 0;
+		const nextSnapshot = new Map<string, UnreadData>();
 
-		const unreadCount = subscriptions.reduce((ret, subscription) => {
-			fireEventUnreadChangedBySubscription(subscription);
+		for (const subscription of subscriptions) {
+			const { rid, unread: unreadValue, alert, unreadAlert: subscriptionUnreadAlert } = subscription;
+			const prev = prevSubsRef.current.get(rid);
+			// Emit per-sub event only if something that influences unread UI changed.
+			if (!prev || prev.unread !== unreadValue || prev.alert !== alert || prev.unreadAlert !== subscriptionUnreadAlert) {
+				fireEventUnreadChangedBySubscription(subscription);
+			}
+			nextSnapshot.set(rid, { unread: unreadValue, alert, unreadAlert: subscriptionUnreadAlert });
 
-			if (subscription.alert || subscription.unread > 0) {
-				// Increment the total unread count.
-				if (subscription.alert === true && subscription.unreadAlert !== 'nothing') {
-					if (subscription.unreadAlert === 'all' || unreadAlertEnabled !== false) {
-						unreadAlert = '•';
+			if (alert || unreadValue > 0) {
+				if (alert === true && subscriptionUnreadAlert !== 'nothing') {
+					if (subscriptionUnreadAlert === 'all' || unreadAlertEnabled !== false) {
+						badgeIndicator = '•';
 					}
 				}
-				return ret + subscription.unread;
+				unreadCount += unreadValue;
 			}
-			return ret;
-		}, 0);
+		}
+
+		prevSubsRef.current = nextSnapshot; // swap snapshot
 
 		if (unreadCount > 0) {
-			if (unreadCount > 999) {
-				setUnread('999+');
-			} else {
-				setUnread(unreadCount);
-			}
-		} else if (unreadAlert !== false) {
-			setUnread(unreadAlert);
+			setUnread(unreadCount > 999 ? '999+' : unreadCount);
+		} else if (badgeIndicator !== false) {
+			setUnread(badgeIndicator);
 		} else {
 			setUnread('');
 		}
+
 		fireEventUnreadChanged(unreadCount);
-	}, [setUnread, unread, subscriptions, unreadAlertEnabled, fireEventUnreadChangedBySubscription, fireEventUnreadChanged]);
+	}, [setUnread, subscriptions, unreadAlertEnabled, fireEventUnreadChangedBySubscription, fireEventUnreadChanged]);
 
 	useEffect(() => {
 		updateFavicon(unread);