diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index 2e860137d..4ef548e19 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -433,7 +433,7 @@ func (r *QueryCurrentStateResponse) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
-// QueryMembershipAtEventRequest requests the membership events for a user
+// QueryMembershipAtEventRequest requests the membership event for a user
 // for a list of eventIDs.
 type QueryMembershipAtEventRequest struct {
 	RoomID   string
@@ -443,9 +443,10 @@ type QueryMembershipAtEventRequest struct {
 
 // QueryMembershipAtEventResponse is the response to QueryMembershipAtEventRequest.
 type QueryMembershipAtEventResponse struct {
-	// Memberships is a map from eventID to a list of events (if any). Events that
-	// do not have known state will return an empty array here.
-	Memberships map[string]*gomatrixserverlib.HeaderedEvent `json:"memberships"`
+	// Membership is a map from eventID to membership event. Events that
+	// do not have known state will return a nil event, resulting in a "leave" membership
+	// when calculating history visibility.
+	Membership map[string]*gomatrixserverlib.HeaderedEvent `json:"membership"`
 }
 
 // QueryLeftUsersRequest is a request to calculate users that we (the server) don't share a
diff --git a/roomserver/internal/query/query.go b/roomserver/internal/query/query.go
index b960e0fa8..1083bb237 100644
--- a/roomserver/internal/query/query.go
+++ b/roomserver/internal/query/query.go
@@ -217,7 +217,7 @@ func (r *Queryer) QueryMembershipAtEvent(
 	request *api.QueryMembershipAtEventRequest,
 	response *api.QueryMembershipAtEventResponse,
 ) error {
-	response.Memberships = make(map[string]*gomatrixserverlib.HeaderedEvent)
+	response.Membership = make(map[string]*gomatrixserverlib.HeaderedEvent)
 
 	info, err := r.DB.RoomInfo(ctx, request.RoomID)
 	if err != nil {
@@ -236,16 +236,16 @@ func (r *Queryer) QueryMembershipAtEvent(
 		return fmt.Errorf("requested stateKeyNID for %s was not found", request.UserID)
 	}
 
-	response.Memberships, err = r.DB.GetMembershipForHistoryVisibility(ctx, stateKeyNIDs[request.UserID], info, request.EventIDs...)
+	response.Membership, err = r.DB.GetMembershipForHistoryVisibility(ctx, stateKeyNIDs[request.UserID], info, request.EventIDs...)
 	switch err {
 	case nil:
 		return nil
-	case tables.OptimisationNotSupportedError: // fallthrough
+	case tables.OptimisationNotSupportedError: // fallthrough, slow way of getting the membership events for each event
 	default:
 		return err
 	}
 
-	response.Memberships = make(map[string]*gomatrixserverlib.HeaderedEvent)
+	response.Membership = make(map[string]*gomatrixserverlib.HeaderedEvent)
 	stateEntries, err := helpers.MembershipAtEvent(ctx, r.DB, nil, request.EventIDs, stateKeyNIDs[request.UserID])
 	if err != nil {
 		return fmt.Errorf("unable to get state before event: %w", err)
@@ -270,7 +270,7 @@ func (r *Queryer) QueryMembershipAtEvent(
 	for _, eventID := range request.EventIDs {
 		stateEntry, ok := stateEntries[eventID]
 		if !ok || len(stateEntry) == 0 {
-			response.Memberships[eventID] = nil
+			response.Membership[eventID] = nil
 			continue
 		}
 
@@ -287,10 +287,13 @@ func (r *Queryer) QueryMembershipAtEvent(
 			return fmt.Errorf("unable to get memberships at state: %w", err)
 		}
 
+		// Iterate over all membership events we got. Given we only query the membership for
+		// one user and assuming this user only ever has one membership event associated to
+		// a given event, overwrite any other existing membership events.
 		for i := range memberships {
 			ev := memberships[i]
 			if ev.Type() == gomatrixserverlib.MRoomMember && ev.StateKeyEquals(request.UserID) {
-				response.Memberships[eventID] = ev.Event.Headered(info.RoomVersion)
+				response.Membership[eventID] = ev.Event.Headered(info.RoomVersion)
 			}
 		}
 	}
diff --git a/roomserver/storage/interface.go b/roomserver/storage/interface.go
index 80c449623..998915122 100644
--- a/roomserver/storage/interface.go
+++ b/roomserver/storage/interface.go
@@ -176,6 +176,9 @@ type Database interface {
 	PurgeRoom(ctx context.Context, roomID string) error
 	UpgradeRoom(ctx context.Context, oldRoomID, newRoomID, eventSender string) error
 
+	// GetMembershipForHistoryVisibility queries the membership events for the given eventIDs.
+	// Returns a map from (input) eventID -> membership event. If no membership event is found, returns an empty event, resulting in
+	// a membership of "leave" when calculating history visibility.
 	GetMembershipForHistoryVisibility(
 		ctx context.Context, userNID types.EventStateKeyNID, info *types.RoomInfo, eventIDs ...string,
 	) (map[string]*gomatrixserverlib.HeaderedEvent, error)
diff --git a/roomserver/storage/postgres/events_table.go b/roomserver/storage/postgres/events_table.go
index 9ccf21d95..f4a21c8a7 100644
--- a/roomserver/storage/postgres/events_table.go
+++ b/roomserver/storage/postgres/events_table.go
@@ -71,7 +71,7 @@ CREATE TABLE IF NOT EXISTS roomserver_events (
 );
 
 -- Create an index which helps in resolving membership events (event_type_nid = 5) - (used for history visibility)
-CREATE INDEX IF NOT EXISTS roomserver_events_history_visibility_idx ON roomserver_events (room_nid, event_state_key_nid) where (event_type_nid = 5);
+CREATE INDEX IF NOT EXISTS roomserver_events_memberships_idx ON roomserver_events (room_nid, event_state_key_nid) WHERE (event_type_nid = 5);
 `
 
 const insertEventSQL = "" +
diff --git a/syncapi/internal/history_visibility.go b/syncapi/internal/history_visibility.go
index 2b4ebaa41..87c59e03e 100644
--- a/syncapi/internal/history_visibility.go
+++ b/syncapi/internal/history_visibility.go
@@ -199,7 +199,7 @@ func visibilityForEvents(
 			membershipAtEvent: gomatrixserverlib.Leave, // default to leave, to not expose events by accident
 			visibility:        event.Visibility,
 		}
-		ev, ok := membershipResp.Memberships[eventID]
+		ev, ok := membershipResp.Membership[eventID]
 		if !ok || ev == nil {
 			result[eventID] = vis
 			continue