diff --git a/syncapi/notifier/notifier.go b/syncapi/notifier/notifier.go
index 4e1a3c2a3..44f133388 100644
--- a/syncapi/notifier/notifier.go
+++ b/syncapi/notifier/notifier.go
@@ -45,6 +45,8 @@ type Notifier struct {
 	lastCleanUpTime time.Time
 	// Protects roomIDToJoinedUsers and roomIDToPeekingDevices
 	mapLock *sync.RWMutex
+	// This map is reused to prevent allocations and GC pressure in SharedUsers.
+	_sharedUsers map[string]struct{}
 }
 
 // NewNotifier creates a new notifier set to the given sync position.
@@ -58,12 +60,16 @@ func NewNotifier() *Notifier {
 		streamLock:             &sync.Mutex{},
 		mapLock:                &sync.RWMutex{},
 		lastCleanUpTime:        time.Now(),
+		_sharedUsers:           map[string]struct{}{},
 	}
 }
 
 // SetCurrentPosition sets the current streaming positions.
 // This must be called directly after NewNotifier and initialising the streams.
 func (n *Notifier) SetCurrentPosition(currPos types.StreamingToken) {
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
+
 	n.currPos = currPos
 }
 
@@ -249,13 +255,22 @@ func (n *Notifier) OnNewPresence(
 	n.wakeupUsers(sharedUsers, nil, n.currPos)
 }
 
-func (n *Notifier) SharedUsers(userID string) (sharedUsers []string) {
+func (n *Notifier) SharedUsers(userID string) []string {
 	n.mapLock.RLock()
 	defer n.mapLock.RUnlock()
+	n._sharedUsers[userID] = struct{}{}
 	for roomID, users := range n.roomIDToJoinedUsers {
-		if _, ok := users[userID]; ok {
-			sharedUsers = append(sharedUsers, n.JoinedUsers(roomID)...)
+		if _, ok := users[userID]; !ok {
+			continue
 		}
+		for _, userID := range n.JoinedUsers(roomID) {
+			n._sharedUsers[userID] = struct{}{}
+		}
+	}
+	sharedUsers := make([]string, 0, len(n._sharedUsers)+1)
+	for userID := range n._sharedUsers {
+		sharedUsers = append(sharedUsers, userID)
+		delete(n._sharedUsers, userID)
 	}
 	return sharedUsers
 }
@@ -328,7 +343,7 @@ func (n *Notifier) setUsersJoinedToRooms(roomIDToUserIDs map[string][]string) {
 	// This is just the bulk form of addJoinedUser
 	for roomID, userIDs := range roomIDToUserIDs {
 		if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
-			n.roomIDToJoinedUsers[roomID] = make(userIDSet)
+			n.roomIDToJoinedUsers[roomID] = make(userIDSet, len(userIDs))
 		}
 		for _, userID := range userIDs {
 			n.roomIDToJoinedUsers[roomID].add(userID)
@@ -343,7 +358,7 @@ func (n *Notifier) setPeekingDevices(roomIDToPeekingDevices map[string][]types.P
 	// This is just the bulk form of addPeekingDevice
 	for roomID, peekingDevices := range roomIDToPeekingDevices {
 		if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
-			n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+			n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet, len(peekingDevices))
 		}
 		for _, peekingDevice := range peekingDevices {
 			n.roomIDToPeekingDevices[roomID].add(peekingDevice)
@@ -416,7 +431,7 @@ func (n *Notifier) fetchUserStreams(userID string) []*UserDeviceStream {
 	if !ok {
 		return []*UserDeviceStream{}
 	}
-	streams := []*UserDeviceStream{}
+	streams := make([]*UserDeviceStream, 0, len(user))
 	for _, stream := range user {
 		streams = append(streams, stream)
 	}
@@ -514,10 +529,10 @@ func (n *Notifier) removeEmptyUserStreams() {
 }
 
 // A string set, mainly existing for improving clarity of structs in this file.
-type userIDSet map[string]bool
+type userIDSet map[string]struct{}
 
 func (s userIDSet) add(str string) {
-	s[str] = true
+	s[str] = struct{}{}
 }
 
 func (s userIDSet) remove(str string) {
@@ -534,10 +549,10 @@ func (s userIDSet) values() (vals []string) {
 
 // A set of PeekingDevices, similar to userIDSet
 
-type peekingDeviceSet map[types.PeekingDevice]bool
+type peekingDeviceSet map[types.PeekingDevice]struct{}
 
 func (s peekingDeviceSet) add(d types.PeekingDevice) {
-	s[d] = true
+	s[d] = struct{}{}
 }
 
 // nolint:unused