diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index a514127c..7766bfb1 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -556,6 +556,7 @@ func (t *txnReq) processEvent(ctx context.Context, e *gomatrixserverlib.Event) e
 		},
 		api.DoNotSendToOtherServers,
 		nil,
+		api.InputOptionAsync,
 	)
 }
 
@@ -598,6 +599,7 @@ withNextEvent:
 						SendAsServer: api.DoNotSendToOtherServers,
 					},
 				},
+				api.InputOptionAsync,
 			); err != nil {
 				return fmt.Errorf("api.SendEvents: %w", err)
 			}
@@ -747,6 +749,7 @@ func (t *txnReq) processEventWithMissingState(
 		resolvedState,
 		backwardsExtremity.Headered(roomVersion),
 		t.hadEvents,
+		api.InputOptionAsync,
 	)
 	if err != nil {
 		return fmt.Errorf("api.SendEventWithState: %w", err)
@@ -767,6 +770,7 @@ func (t *txnReq) processEventWithMissingState(
 		append(headeredNewEvents, e.Headered(roomVersion)),
 		api.DoNotSendToOtherServers,
 		nil,
+		api.InputOptionAsync,
 	); err != nil {
 		return fmt.Errorf("api.SendEvents: %w", err)
 	}
diff --git a/roomserver/api/input.go b/roomserver/api/input.go
index 8e6e4ac7..3e8b8662 100644
--- a/roomserver/api/input.go
+++ b/roomserver/api/input.go
@@ -86,6 +86,9 @@ type TransactionID struct {
 // InputRoomEventsRequest is a request to InputRoomEvents
 type InputRoomEventsRequest struct {
 	InputRoomEvents []InputRoomEvent `json:"input_room_events"`
+	// Asynchronous requests will queue up work into the roomserver and
+	// return straight away, rather than waiting for the results.
+	Asynchronous bool `json:"async"`
 }
 
 // InputRoomEventsResponse is a response to InputRoomEvents
diff --git a/roomserver/api/wrapper.go b/roomserver/api/wrapper.go
index 2ebe2f64..8909a70c 100644
--- a/roomserver/api/wrapper.go
+++ b/roomserver/api/wrapper.go
@@ -22,11 +22,18 @@ import (
 	"github.com/matrix-org/util"
 )
 
+type InputOption int
+
+const (
+	InputOptionAsync InputOption = iota
+)
+
 // SendEvents to the roomserver The events are written with KindNew.
 func SendEvents(
 	ctx context.Context, rsAPI RoomserverInternalAPI,
 	kind Kind, events []*gomatrixserverlib.HeaderedEvent,
 	sendAsServer gomatrixserverlib.ServerName, txnID *TransactionID,
+	options ...InputOption,
 ) error {
 	ires := make([]InputRoomEvent, len(events))
 	for i, event := range events {
@@ -38,7 +45,7 @@ func SendEvents(
 			TransactionID: txnID,
 		}
 	}
-	return SendInputRoomEvents(ctx, rsAPI, ires)
+	return SendInputRoomEvents(ctx, rsAPI, ires, options...)
 }
 
 // SendEventWithState writes an event with the specified kind to the roomserver
@@ -47,7 +54,7 @@ func SendEvents(
 func SendEventWithState(
 	ctx context.Context, rsAPI RoomserverInternalAPI, kind Kind,
 	state *gomatrixserverlib.RespState, event *gomatrixserverlib.HeaderedEvent,
-	haveEventIDs map[string]bool,
+	haveEventIDs map[string]bool, options ...InputOption,
 ) error {
 	outliers, err := state.Events()
 	if err != nil {
@@ -79,14 +86,25 @@ func SendEventWithState(
 		StateEventIDs: stateEventIDs,
 	})
 
-	return SendInputRoomEvents(ctx, rsAPI, ires)
+	return SendInputRoomEvents(ctx, rsAPI, ires, options...)
 }
 
 // SendInputRoomEvents to the roomserver.
 func SendInputRoomEvents(
-	ctx context.Context, rsAPI RoomserverInternalAPI, ires []InputRoomEvent,
+	ctx context.Context, rsAPI RoomserverInternalAPI,
+	ires []InputRoomEvent, options ...InputOption,
 ) error {
-	request := InputRoomEventsRequest{InputRoomEvents: ires}
+	async := false
+	for _, opt := range options {
+		switch opt {
+		case InputOptionAsync:
+			async = true
+		}
+	}
+	request := InputRoomEventsRequest{
+		InputRoomEvents: ires,
+		Asynchronous:    async,
+	}
 	var response InputRoomEventsResponse
 	rsAPI.InputRoomEvents(ctx, &request, &response)
 	return response.Err()
diff --git a/roomserver/internal/input/input.go b/roomserver/internal/input/input.go
index b8279a86..b64fc1fa 100644
--- a/roomserver/internal/input/input.go
+++ b/roomserver/internal/input/input.go
@@ -166,17 +166,19 @@ func (r *Inputer) InputRoomEvents(
 		worker.input.push(tasks[i])
 	}
 
-	// Wait for all of the workers to return results about our tasks.
-	wg.Wait()
+	if !request.Asynchronous {
+		// Wait for all of the workers to return results about our tasks.
+		wg.Wait()
 
-	// If any of the tasks returned an error, we should probably report
-	// that back to the caller.
-	for _, task := range tasks {
-		if task.err != nil {
-			response.ErrMsg = task.err.Error()
-			_, rejected := task.err.(*gomatrixserverlib.NotAllowed)
-			response.NotAllowed = rejected
-			return
+		// If any of the tasks returned an error, we should probably report
+		// that back to the caller.
+		for _, task := range tasks {
+			if task.err != nil {
+				response.ErrMsg = task.err.Error()
+				_, rejected := task.err.(*gomatrixserverlib.NotAllowed)
+				response.NotAllowed = rejected
+				return
+			}
 		}
 	}
 }