diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index 5d5ea310d..5f20b2d8e 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -19,6 +19,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"net/http"
+	"sync"
 	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
@@ -705,6 +706,20 @@ Event:
 	return nil, nil
 }
 
+func (t *txnReq) lookupMissingStateViaState(ctx context.Context, roomID, eventID string, roomVersion gomatrixserverlib.RoomVersion) (
+	respState *gomatrixserverlib.RespState, err error) {
+	state, err := t.federation.LookupState(ctx, t.Origin, roomID, eventID, roomVersion)
+	if err != nil {
+		return nil, err
+	}
+	// Check that the returned state is valid.
+	if err := state.Check(ctx, t.keys, nil); err != nil {
+		return nil, err
+	}
+	return &state, nil
+}
+
+// nolint:gocyclo
 func (t *txnReq) lookupMissingStateViaStateIDs(ctx context.Context, roomID, eventID string, roomVersion gomatrixserverlib.RoomVersion) (
 	*gomatrixserverlib.RespState, error) {
 	util.GetLogger(ctx).Infof("lookupMissingStateViaStateIDs %s", eventID)
@@ -742,27 +757,90 @@ func (t *txnReq) lookupMissingStateViaStateIDs(ctx context.Context, roomID, even
 		}
 	}
 
+	concurrentRequests := 8
+	missingCount := len(missing)
+
+	// If over 50% of the auth/state events from /state_ids are missing
+	// then we'll just call /state instead, otherwise we'll just end up
+	// hammering the remote side with /event requests unnecessarily.
+	if missingCount > concurrentRequests && missingCount > len(wantIDs)/2 {
+		util.GetLogger(ctx).WithFields(logrus.Fields{
+			"missing":           missingCount,
+			"event_id":          eventID,
+			"room_id":           roomID,
+			"total_state":       len(stateIDs.StateEventIDs),
+			"total_auth_events": len(stateIDs.AuthEventIDs),
+		}).Info("Fetching all state at event")
+		return t.lookupMissingStateViaState(ctx, roomID, eventID, roomVersion)
+	}
+
 	util.GetLogger(ctx).WithFields(logrus.Fields{
-		"missing":           len(missing),
-		"event_id":          eventID,
-		"room_id":           roomID,
-		"total_state":       len(stateIDs.StateEventIDs),
-		"total_auth_events": len(stateIDs.AuthEventIDs),
+		"missing":             missingCount,
+		"event_id":            eventID,
+		"room_id":             roomID,
+		"total_state":         len(stateIDs.StateEventIDs),
+		"total_auth_events":   len(stateIDs.AuthEventIDs),
+		"concurrent_requests": concurrentRequests,
 	}).Info("Fetching missing state at event")
 
+	// Create a queue containing all of the missing event IDs that we want
+	// to retrieve.
+	pending := make(chan string, missingCount)
 	for missingEventID := range missing {
+		pending <- missingEventID
+	}
+	close(pending)
+
+	// Define how many workers we should start to do this.
+	if missingCount < concurrentRequests {
+		concurrentRequests = missingCount
+	}
+
+	// Create the wait group.
+	var fetchgroup sync.WaitGroup
+	fetchgroup.Add(concurrentRequests)
+
+	// This is the only place where we'll write to t.haveEvents from
+	// multiple goroutines, and everywhere else is blocked on this
+	// synchronous function anyway.
+	var haveEventsMutex sync.Mutex
+
+	// Define what we'll do in order to fetch the missing event ID.
+	fetch := func(missingEventID string) {
 		var h *gomatrixserverlib.HeaderedEvent
 		h, err = t.lookupEvent(ctx, roomVersion, missingEventID, false)
 		switch err.(type) {
 		case verifySigError:
-			continue
+			break
 		case nil:
-			// do nothing
+			break
 		default:
-			return nil, err
+			util.GetLogger(ctx).WithFields(logrus.Fields{
+				"event_id": missingEventID,
+				"room_id":  roomID,
+			}).Info("Failed to fetch missing event")
+			return
 		}
+		haveEventsMutex.Lock()
 		t.haveEvents[h.EventID()] = h
+		haveEventsMutex.Unlock()
 	}
+
+	// Create the worker.
+	worker := func(ch <-chan string) {
+		defer fetchgroup.Done()
+		for missingEventID := range ch {
+			fetch(missingEventID)
+		}
+	}
+
+	// Start the workers.
+	for i := 0; i < concurrentRequests; i++ {
+		go worker(pending)
+	}
+
+	// Wait for the workers to finish.
+	fetchgroup.Wait()
 	resp, err := t.createRespStateFromStateIDs(stateIDs)
 	return resp, err
 }