diff --git a/go.mod b/go.mod
index b104b4192..a416ec98f 100644
--- a/go.mod
+++ b/go.mod
@@ -39,7 +39,7 @@ require (
 	github.com/matrix-org/go-http-js-libp2p v0.0.0-20200518170932-783164aeeda4
 	github.com/matrix-org/go-sqlite3-js v0.0.0-20210709140738-b0d1ba599a6d
 	github.com/matrix-org/gomatrix v0.0.0-20210324163249-be2af5ef2e16
-	github.com/matrix-org/gomatrixserverlib v0.0.0-20220214133635-20632dd262ed
+	github.com/matrix-org/gomatrixserverlib v0.0.0-20220224170509-f6ab9c54d052
 	github.com/matrix-org/pinecone v0.0.0-20220223104432-0f0afd1a46aa
 	github.com/matrix-org/util v0.0.0-20200807132607-55161520e1d4
 	github.com/mattn/go-sqlite3 v1.14.10
diff --git a/go.sum b/go.sum
index 23ffe8b85..ecb0496bc 100644
--- a/go.sum
+++ b/go.sum
@@ -983,8 +983,8 @@ github.com/matrix-org/go-sqlite3-js v0.0.0-20210709140738-b0d1ba599a6d/go.mod h1
 github.com/matrix-org/gomatrix v0.0.0-20190528120928-7df988a63f26/go.mod h1:3fxX6gUjWyI/2Bt7J1OLhpCzOfO/bB3AiX0cJtEKud0=
 github.com/matrix-org/gomatrix v0.0.0-20210324163249-be2af5ef2e16 h1:ZtO5uywdd5dLDCud4r0r55eP4j9FuUNpl60Gmntcop4=
 github.com/matrix-org/gomatrix v0.0.0-20210324163249-be2af5ef2e16/go.mod h1:/gBX06Kw0exX1HrwmoBibFA98yBk/jxKpGVeyQbff+s=
-github.com/matrix-org/gomatrixserverlib v0.0.0-20220214133635-20632dd262ed h1:R8EiLWArq7KT96DrUq1xq9scPh8vLwKKeCTnORPyjhU=
-github.com/matrix-org/gomatrixserverlib v0.0.0-20220214133635-20632dd262ed/go.mod h1:qFvhfbQ5orQxlH9vCiFnP4dW27xxnWHdNUBKyj/fbiY=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20220224170509-f6ab9c54d052 h1:+4Q/JQ3fGgA7sIHaLMlqREX8yEpsI+HlVoW9WId7SNc=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20220224170509-f6ab9c54d052/go.mod h1:+WF5InseAMgi1fTnU46JH39IDpEvLep0fDzx9LDf2Bo=
 github.com/matrix-org/pinecone v0.0.0-20220223104432-0f0afd1a46aa h1:rMYFNVto66gp+eWS8XAUzgp4m0qmUBid6l1HX3mHstk=
 github.com/matrix-org/pinecone v0.0.0-20220223104432-0f0afd1a46aa/go.mod h1:r6dsL+ylE0yXe/7zh8y/Bdh6aBYI1r+u4yZni9A4iyk=
 github.com/matrix-org/util v0.0.0-20190711121626-527ce5ddefc7/go.mod h1:vVQlW/emklohkZnOPwD3LrZUBqdfsbiyO3p1lNV8F6U=
diff --git a/roomserver/state/state.go b/roomserver/state/state.go
index e5f69521e..187b996cd 100644
--- a/roomserver/state/state.go
+++ b/roomserver/state/state.go
@@ -814,6 +814,7 @@ func (v *StateResolution) resolveConflictsV2(
 	// events may be duplicated across these sets but that's OK.
 	authSets := make(map[string][]*gomatrixserverlib.Event, len(conflicted))
 	authEvents := make([]*gomatrixserverlib.Event, 0, estimate*3)
+	gotAuthEvents := make(map[string]struct{}, estimate*3)
 	authDifference := make([]*gomatrixserverlib.Event, 0, estimate)
 
 	// For each conflicted event, let's try and get the needed auth events.
@@ -850,9 +851,22 @@ func (v *StateResolution) resolveConflictsV2(
 		if err != nil {
 			return nil, err
 		}
-		authEvents = append(authEvents, authSets[key]...)
+
+		// Only add auth events into the authEvents slice once, otherwise the
+		// check for the auth difference can become expensive and produce
+		// duplicate entries, which just waste memory and CPU time.
+		for _, event := range authSets[key] {
+			if _, ok := gotAuthEvents[event.EventID()]; !ok {
+				authEvents = append(authEvents, event)
+				gotAuthEvents[event.EventID()] = struct{}{}
+			}
+		}
 	}
 
+	// Kill the reference to this so that the GC may pick it up, since we no
+	// longer need this after this point.
+	gotAuthEvents = nil // nolint:ineffassign
+
 	// This function helps us to work out whether an event exists in one of the
 	// auth sets.
 	isInAuthList := func(k string, event *gomatrixserverlib.Event) bool {
@@ -866,11 +880,12 @@ func (v *StateResolution) resolveConflictsV2(
 
 	// This function works out if an event exists in all of the auth sets.
 	isInAllAuthLists := func(event *gomatrixserverlib.Event) bool {
-		found := true
 		for k := range authSets {
-			found = found && isInAuthList(k, event)
+			if !isInAuthList(k, event) {
+				return false
+			}
 		}
-		return found
+		return true
 	}
 
 	// Look through all of the auth events that we've been given and work out if