diff --git a/internal/caching/cache_lazy_load_members.go b/internal/caching/cache_lazy_load_members.go
index 37c1802cb..0d7009c94 100644
--- a/internal/caching/cache_lazy_load_members.go
+++ b/internal/caching/cache_lazy_load_members.go
@@ -4,61 +4,32 @@ import (
 	userapi "github.com/matrix-org/dendrite/userapi/api"
 )
 
+type lazyLoadingCacheKey struct {
+	UserID       string // the user we're querying on behalf of
+	DeviceID     string // the user we're querying on behalf of
+	RoomID       string // the room in question
+	TargetUserID string // the user whose membership we're asking about
+}
+
 type LazyLoadCache interface {
 	StoreLazyLoadedUser(device *userapi.Device, roomID, userID, eventID string)
 	IsLazyLoadedUserCached(device *userapi.Device, roomID, userID string) (string, bool)
 }
 
-func (c Caches) lazyLoadCacheForUser(device *userapi.Device) (*RistrettoCachePartition[string, any], error) {
-	return nil, nil
-	/*
-		cacheName := fmt.Sprintf("%s/%s", device.UserID, device.ID)
-		userCache, ok := c.LazyLoading.Get(cacheName)
-		if ok && userCache != nil {
-			if cache, ok := userCache.(*InMemoryLRUCachePartition); ok {
-				return cache, nil
-			}
-		}
-		cache, err := NewInMemoryLRUCachePartition(
-			LazyLoadCacheName,
-			LazyLoadCacheMutable,
-			LazyLoadCacheMaxUserEntries,
-			LazyLoadCacheMaxAge,
-			false,
-		)
-		if err != nil {
-			return nil, err
-		}
-		c.LazyLoading.Set(cacheName, cache)
-		go cacheCleaner(cache)
-		return cache, nil
-	*/
-}
-
 func (c Caches) StoreLazyLoadedUser(device *userapi.Device, roomID, userID, eventID string) {
-	/*
-		cache, err := c.lazyLoadCacheForUser(device)
-		if err != nil {
-			return
-		}
-		cacheKey := fmt.Sprintf("%s/%s/%s/%s", device.UserID, device.ID, roomID, userID)
-		cache.Set(cacheKey, eventID)
-	*/
+	c.LazyLoading.Set(lazyLoadingCacheKey{
+		UserID:       device.UserID,
+		DeviceID:     device.ID,
+		RoomID:       roomID,
+		TargetUserID: userID,
+	}, eventID)
 }
 
 func (c Caches) IsLazyLoadedUserCached(device *userapi.Device, roomID, userID string) (string, bool) {
-	return "", false
-	/*
-		cache, err := c.lazyLoadCacheForUser(device)
-		if err != nil {
-			return "", false
-		}
-
-		cacheKey := fmt.Sprintf("%s/%s/%s/%s", device.UserID, device.ID, roomID, userID)
-		val, ok := cache.Get(cacheKey)
-		if !ok {
-			return "", ok
-		}
-		return val.(string), ok
-	*/
+	return c.LazyLoading.Get(lazyLoadingCacheKey{
+		UserID:       device.UserID,
+		DeviceID:     device.ID,
+		RoomID:       roomID,
+		TargetUserID: userID,
+	})
 }
diff --git a/internal/caching/caches.go b/internal/caching/caches.go
index 1a5133790..5bc851775 100644
--- a/internal/caching/caches.go
+++ b/internal/caching/caches.go
@@ -18,7 +18,7 @@ type Caches struct {
 	FederationPDUs     Cache[int64, *gomatrixserverlib.HeaderedEvent]
 	FederationEDUs     Cache[int64, *gomatrixserverlib.EDU]
 	SpaceSummaryRooms  Cache[string, gomatrixserverlib.MSC2946SpacesResponse]
-	LazyLoading        Cache[string, any]
+	LazyLoading        Cache[lazyLoadingCacheKey, string]
 }
 
 // Cache is the interface that an implementation must satisfy.
@@ -30,7 +30,7 @@ type Cache[K keyable, T any] interface {
 
 type keyable interface {
 	// from https://github.com/dgraph-io/ristretto/blob/8e850b710d6df0383c375ec6a7beae4ce48fc8d5/z/z.go#L34
-	uint64 | string | []byte | byte | int | int32 | uint32 | int64
+	uint64 | string | []byte | byte | int | int32 | uint32 | int64 | lazyLoadingCacheKey
 }
 
 type costable interface {
diff --git a/internal/caching/impl_ristretto.go b/internal/caching/impl_ristretto.go
index 4c5e5a6ef..6964d5ded 100644
--- a/internal/caching/impl_ristretto.go
+++ b/internal/caching/impl_ristretto.go
@@ -111,7 +111,7 @@ func NewRistrettoCache(maxCost config.DataUnit, maxAge time.Duration, enableProm
 			Mutable: true,
 			MaxAge:  maxAge,
 		},
-		LazyLoading: &RistrettoCachePartition[string, any]{ // TODO: type
+		LazyLoading: &RistrettoCachePartition[lazyLoadingCacheKey, string]{
 			cache:   cache,
 			Prefix:  lazyLoadingCache,
 			Mutable: true,