diff --git a/roomserver/api/perform.go b/roomserver/api/perform.go
index 2818efaa3..22346df80 100644
--- a/roomserver/api/perform.go
+++ b/roomserver/api/perform.go
@@ -93,11 +93,19 @@ type PerformBackfillRequest struct {
 
 // PrevEventIDs returns the prev_event IDs of all backwards extremities, de-duplicated in a lexicographically sorted order.
 func (r *PerformBackfillRequest) PrevEventIDs() []string {
-	var prevEventIDs []string
+	// Collect 1k eventIDs, if possible, they may be cleared out below
+	prevEventIDs := make([]string, 0, len(r.BackwardsExtremities)*3)
 	for _, pes := range r.BackwardsExtremities {
 		prevEventIDs = append(prevEventIDs, pes...)
+		if len(prevEventIDs) > 1000 {
+			break
+		}
 	}
 	prevEventIDs = util.UniqueStrings(prevEventIDs)
+	// If we still have > 100 eventIDs, only return the first 100
+	if len(prevEventIDs) > 100 {
+		return prevEventIDs[:100]
+	}
 	return prevEventIDs
 }
 
diff --git a/roomserver/api/perform_test.go b/roomserver/api/perform_test.go
new file mode 100644
index 000000000..4a6e9238f
--- /dev/null
+++ b/roomserver/api/perform_test.go
@@ -0,0 +1,53 @@
+package api
+
+import (
+	"fmt"
+	"math/rand"
+	"testing"
+)
+
+func BenchmarkPrevEventIDs(b *testing.B) {
+	for _, x := range []int64{1, 2, 4, 10, 50, 100, 300, 500, 800, 1000, 2000, 3000, 5000, 10000} {
+		benchPrevEventIDs(b, int(x))
+	}
+}
+
+func benchPrevEventIDs(b *testing.B, count int) {
+	b.Run(fmt.Sprintf("Benchmark%d", count), func(b *testing.B) {
+		bwExtrems := generateBackwardsExtremities(b, count)
+		backfiller := PerformBackfillRequest{
+			BackwardsExtremities: bwExtrems,
+		}
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			prevIDs := backfiller.PrevEventIDs()
+			_ = prevIDs
+		}
+	})
+}
+
+func generateBackwardsExtremities(b *testing.B, count int) map[string][]string {
+	b.Helper()
+	result := make(map[string][]string, count)
+	for i := 0; i < count; i++ {
+		eventID := randomEventId(int64(i), 10)
+		result[eventID] = []string{
+			randomEventId(int64(i+1), 10),
+			randomEventId(int64(i+2), 10),
+			randomEventId(int64(i+3), 10),
+		}
+	}
+	return result
+}
+
+const alphanumerics = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+
+// RandomString generates a pseudo-random string of length n.
+func randomEventId(src int64, n int) string {
+	randSrc := rand.NewSource(src)
+	b := make([]byte, n)
+	for i := range b {
+		b[i] = alphanumerics[randSrc.Int63()%int64(len(alphanumerics))]
+	}
+	return string(b)
+}