diff --git a/src/stores/RoomListStore.js b/src/stores/RoomListStore.js
index 8909dbb489..2b70d53b59 100644
--- a/src/stores/RoomListStore.js
+++ b/src/stores/RoomListStore.js
@@ -54,6 +54,24 @@ class RoomListStore extends Store {
                 "im.vector.fake.archived": [],
             },
             ready: false,
+
+            // The room cache stores a mapping of roomId to cache record.
+            // Each cache record is a key/value pair for various bits of
+            // data used to sort the room list. Currently this stores the
+            // following bits of informations:
+            //   "timestamp":     number, The timestamp of the last relevant
+            //                    event in the room.
+            //   "notifications": boolean, Whether or not the user has been
+            //                    highlighted on any unread events.
+            //   "unread":        boolean, Whether or not the user has any
+            //                    unread events.
+            //
+            // All of the cached values are lazily loaded on read in the
+            // recents comparator. When an event is received for a particular
+            // room, all the cached values are invalidated - forcing the
+            // next read to set new values. The entries do not expire on
+            // their own.
+            roomCache: {},
         };
     }
 
@@ -85,6 +103,8 @@ class RoomListStore extends Store {
                     !payload.isLiveUnfilteredRoomTimelineEvent ||
                     !this._eventTriggersRecentReorder(payload.event)
                 ) break;
+
+                this._clearCachedRoomState(payload.event.getRoomId());
                 this._generateRoomLists();
             }
             break;
@@ -112,6 +132,8 @@ class RoomListStore extends Store {
                 if (liveTimeline !== eventTimeline ||
                     !this._eventTriggersRecentReorder(payload.event)
                 ) break;
+
+                this._clearCachedRoomState(payload.event.getRoomId());
                 this._generateRoomLists();
             }
             break;
@@ -217,11 +239,18 @@ class RoomListStore extends Store {
             }
         });
 
+        // Note: we check the settings up here instead of in the forEach or
+        // in the _recentsComparator to avoid hitting the SettingsStore a few
+        // thousand times.
+        const pinUnread = SettingsStore.getValue("pinUnreadRooms");
+        const pinMentioned = SettingsStore.getValue("pinMentionedRooms");
         Object.keys(lists).forEach((listKey) => {
             let comparator;
             switch (RoomListStore._listOrders[listKey]) {
                 case "recent":
-                    comparator = this._recentsComparator;
+                    comparator = (roomA, roomB) => {
+                        return this._recentsComparator(roomA, roomB, pinUnread, pinMentioned);
+                    };
                     break;
                 case "manual":
                 default:
@@ -237,6 +266,44 @@ class RoomListStore extends Store {
         });
     }
 
+    _updateCachedRoomState(roomId, type, value) {
+        const roomCache = this._state.roomCache;
+        if (!roomCache[roomId]) roomCache[roomId] = {};
+
+        if (value) roomCache[roomId][type] = value;
+        else delete roomCache[roomId][type];
+
+        this._setState({roomCache});
+    }
+
+    _clearCachedRoomState(roomId) {
+        const roomCache = this._state.roomCache;
+        delete roomCache[roomId];
+        this._setState({roomCache});
+    }
+
+    _getRoomState(room, type) {
+        const roomId = room.roomId;
+        const roomCache = this._state.roomCache;
+        if (roomCache[roomId] && typeof roomCache[roomId][type] !== 'undefined') {
+            return roomCache[roomId][type];
+        }
+
+        if (type === "timestamp") {
+            const ts = this._tsOfNewestEvent(room);
+            this._updateCachedRoomState(roomId, "timestamp", ts);
+            return ts;
+        } else if (type === "unread") {
+            const unread = room.getUnreadNotificationCount() > 0;
+            this._updateCachedRoomState(roomId, "unread", unread);
+            return unread;
+        } else if (type === "notifications") {
+            const notifs = room.getUnreadNotificationCount("highlight") > 0;
+            this._updateCachedRoomState(roomId, "notifications", notifs);
+            return notifs;
+        } else throw new Error("Unrecognized room cache type: " + type);
+    }
+
     _eventTriggersRecentReorder(ev) {
         return ev.getTs() && (
             Unread.eventTriggersUnreadCount(ev) ||
@@ -262,34 +329,40 @@ class RoomListStore extends Store {
         }
     }
 
-    _recentsComparator(roomA, roomB) {
-        const pinUnread = SettingsStore.getValue("pinUnreadRooms");
-        const pinMentioned = SettingsStore.getValue("pinMentionedRooms");
-
+    _recentsComparator(roomA, roomB, pinUnread, pinMentioned) {
         // We try and set the ordering to be Mentioned > Unread > Recent
-        // assuming the user has the right settings, of course
+        // assuming the user has the right settings, of course.
+
+        const timestampA = this._getRoomState(roomA, "timestamp");
+        const timestampB = this._getRoomState(roomB, "timestamp");
+        const timestampDiff = timestampB - timestampA;
 
         if (pinMentioned) {
-             const mentionsA = roomA.getUnreadNotificationCount("highlight") > 0;
-             const mentionsB = roomB.getUnreadNotificationCount("highlight") > 0;
-             if (mentionsA && !mentionsB) return -1;
-             if (!mentionsA && mentionsB) return 1;
-             if (mentionsA && mentionsB) return 0;
-             // If neither have mentions, fall through to remaining checks
+            const mentionsA = this._getRoomState(roomA, "notifications");
+            const mentionsB = this._getRoomState(roomB, "notifications");
+            if (mentionsA && !mentionsB) return -1;
+            if (!mentionsA && mentionsB) return 1;
+
+            // If they both have notifications, sort by timestamp.
+            // If neither have notifications (the fourth check not shown
+            // here), then try and sort by unread messages and finally by
+            // timestamp.
+            if (mentionsA && mentionsB) return timestampDiff;
         }
 
         if (pinUnread) {
-            const unreadA = Unread.doesRoomHaveUnreadMessages(roomA);
-            const unreadB = Unread.doesRoomHaveUnreadMessages(roomB);
+            const unreadA = this._getRoomState(roomA, "unread");
+            const unreadB = this._getRoomState(roomB, "unread");
             if (unreadA && !unreadB) return -1;
             if (!unreadA && unreadB) return 1;
-            if (unreadA && unreadB) return 0;
-            // If neither have unread messages, fall through to remaining checks
+
+            // If they both have unread messages, sort by timestamp
+            // If nether have unread message (the fourth check not shown
+            // here), then just sort by timestamp anyways.
+            if (unreadA && unreadB) return timestampDiff;
         }
 
-        // XXX: We could use a cache here and update it when we see new
-        // events that trigger a reorder
-        return this._tsOfNewestEvent(roomB) - this._tsOfNewestEvent(roomA);
+        return timestampDiff;
     }
 
     _lexicographicalComparator(roomA, roomB) {