# UNDF: UNDF-2026-000000263
--- a/apps/meteor/app/lib/server/lib/sendNotificationsOnMessage.ts
+++ b/apps/meteor/app/lib/server/lib/sendNotificationsOnMessage.ts
@@ -73,7 +73,8 @@ export const sendNotification = async ({
 	if (subscription.u._id === sender._id) {
 		return;
 	}

-	const hasMentionToUser = mentionIds.includes(subscription.u._id);
+	const hasMentionToUser = mentionIdSet.has(subscription.u._id);

 	// mute group notifications (@here and @all) if not directly mentioned as well
@@ -57,7 +57,8 @@ export const sendNotification = async ({
 	subscription,
 	sender,
 	hasReplyToThread,
 	hasMentionToAll,
 	hasMentionToHere,
 	message,
 	notificationMessage,
 	room,
-	mentionIds,
+	mentionIdSet,
 	disableAllMessageNotifications,
 }: {
 	subscription: SubscriptionAggregation;
 	sender: Pick<IUser, '_id' | 'name' | 'username'>;
 	hasReplyToThread: boolean;
 	hasMentionToAll: boolean;
 	hasMentionToHere: boolean;
 	message: AtLeast<IMessage, '_id' | 'u' | 'msg' | 't' | 'attachments'>;
 	notificationMessage: string;
 	room: IRoom;
-	mentionIds: string[];
+	mentionIdSet: Set<string>;
 	disableAllMessageNotifications: boolean;
@@ -360,13 +360,15 @@ export async function sendMessageNotifications(message: IMessage, room: IRoom,
 	const subscriptions = await Subscriptions.col.aggregate<SubscriptionAggregation>([{ $match: query }, lookup, filter, project]).toArray();

+	const mentionIdSet = new Set(mentionIds);
+	const usersInThreadSet = new Set(usersInThread ?? []);
+
 	subscriptions.forEach(
 		(subscription) =>
 			void sendNotification({
 				subscription,
 				sender,
 				hasMentionToAll,
 				hasMentionToHere,
 				message,
 				notificationMessage,
 				room,
-				mentionIds,
+				mentionIdSet,
 				disableAllMessageNotifications,
-				hasReplyToThread: usersInThread?.includes(subscription.u._id),
+				hasReplyToThread: usersInThreadSet.has(subscription.u._id),
 			}),
 	);