diff --git a/roomserver/acls/acls.go b/roomserver/acls/acls.go
index 660f4f3bb..299824327 100644
--- a/roomserver/acls/acls.go
+++ b/roomserver/acls/acls.go
@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"net"
 	"regexp"
+	"runtime"
 	"strings"
 	"sync"
 
@@ -41,8 +42,9 @@ type ServerACLDatabase interface {
 }
 
 type ServerACLs struct {
-	acls      map[string]*serverACL // room ID -> ACL
-	aclsMutex sync.RWMutex          // protects the above
+	acls             map[string]*serverACL // room ID -> ACL
+	aclsMutex        sync.RWMutex          // protects the above
+	compileRegexFunc func(orig string) (*regexp.Regexp, error)
 }
 
 func NewServerACLs(db ServerACLDatabase) *ServerACLs {
@@ -50,6 +52,8 @@ func NewServerACLs(db ServerACLDatabase) *ServerACLs {
 	acls := &ServerACLs{
 		acls: make(map[string]*serverACL),
 	}
+	// temporary regex function to use on start up
+	acls.compileRegexFunc = cachedCompileACLRegex()
 	// Look up all of the rooms that the current state server knows about.
 	rooms, err := db.GetKnownRooms(ctx)
 	if err != nil {
@@ -67,6 +71,15 @@ func NewServerACLs(db ServerACLDatabase) *ServerACLs {
 	for _, event := range events {
 		acls.OnServerACLUpdate(event)
 	}
+
+	// All regexes are built, replace the function to use.
+	acls.compileRegexFunc = compileACLRegex
+
+	// Force a GC, as we potentially allocated hundreds of regexes
+	// which can now be GCed, since we replaced the regex function
+	// containing the cache.
+	runtime.GC()
+
 	return acls
 }
 
@@ -89,6 +102,25 @@ func compileACLRegex(orig string) (*regexp.Regexp, error) {
 	return regexp.Compile(escaped)
 }
 
+// cachedCompileACLRegex is a wrapper around compileACLRegex with added caching
+func cachedCompileACLRegex() func(string) (*regexp.Regexp, error) {
+	// Be generous when creating the cache, as in reality
+	// there are hundreds of servers in an ACL.
+	cache := make(map[string]*regexp.Regexp, 100)
+	return func(orig string) (*regexp.Regexp, error) {
+		re, ok := cache[orig]
+		if ok {
+			return re, nil
+		}
+		re, err := compileACLRegex(orig)
+		if err != nil {
+			return nil, err
+		}
+		cache[orig] = re
+		return re, nil
+	}
+}
+
 func (s *ServerACLs) OnServerACLUpdate(strippedEvent tables.StrippedEvent) {
 	acls := &serverACL{}
 	if err := json.Unmarshal([]byte(strippedEvent.ContentValue), &acls.ServerACL); err != nil {
@@ -100,14 +132,14 @@ func (s *ServerACLs) OnServerACLUpdate(strippedEvent tables.StrippedEvent) {
 	// special characters and then replace * and ? with their regex counterparts.
 	// https://matrix.org/docs/spec/client_server/r0.6.1#m-room-server-acl
 	for _, orig := range acls.Allowed {
-		if expr, err := compileACLRegex(orig); err != nil {
+		if expr, err := s.compileRegexFunc(orig); err != nil {
 			logrus.WithError(err).Errorf("Failed to compile allowed regex")
 		} else {
 			acls.allowedRegexes = append(acls.allowedRegexes, expr)
 		}
 	}
 	for _, orig := range acls.Denied {
-		if expr, err := compileACLRegex(orig); err != nil {
+		if expr, err := s.compileRegexFunc(orig); err != nil {
 			logrus.WithError(err).Errorf("Failed to compile denied regex")
 		} else {
 			acls.deniedRegexes = append(acls.deniedRegexes, expr)
diff --git a/roomserver/acls/acls_test.go b/roomserver/acls/acls_test.go
index 9fb6a5581..bd59db4dd 100644
--- a/roomserver/acls/acls_test.go
+++ b/roomserver/acls/acls_test.go
@@ -15,8 +15,14 @@
 package acls
 
 import (
+	"context"
 	"regexp"
 	"testing"
+
+	"github.com/matrix-org/dendrite/roomserver/storage/tables"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/gomatrixserverlib/spec"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestOpenACLsWithBlacklist(t *testing.T) {
@@ -103,3 +109,45 @@ func TestDefaultACLsWithWhitelist(t *testing.T) {
 		t.Fatal("Expected qux.com:4567 to be allowed but wasn't")
 	}
 }
+
+var (
+	content1 = `{"allow":["*"],"allow_ip_literals":false,"deny":["hello.world", "*.hello.world"]}`
+)
+
+type dummyACLDB struct{}
+
+func (d dummyACLDB) GetKnownRooms(ctx context.Context) ([]string, error) {
+	return []string{"1", "2"}, nil
+}
+
+func (d dummyACLDB) GetBulkStateContent(ctx context.Context, roomIDs []string, tuples []gomatrixserverlib.StateKeyTuple, allowWildcards bool) ([]tables.StrippedEvent, error) {
+	return []tables.StrippedEvent{
+		{
+			RoomID:       "1",
+			ContentValue: content1,
+		},
+		{
+			RoomID:       "2",
+			ContentValue: content1,
+		},
+	}, nil
+}
+
+func TestCachedRegex(t *testing.T) {
+	db := dummyACLDB{}
+	wantBannedServer := spec.ServerName("hello.world")
+
+	acls := NewServerACLs(db)
+
+	// Check that hello.world is banned in room 1
+	banned := acls.IsServerBannedFromRoom(wantBannedServer, "1")
+	assert.True(t, banned)
+
+	// Check that hello.world is banned in room 2
+	banned = acls.IsServerBannedFromRoom(wantBannedServer, "2")
+	assert.True(t, banned)
+
+	// Check that matrix.hello.world is banned in room 2
+	banned = acls.IsServerBannedFromRoom("matrix."+wantBannedServer, "2")
+	assert.True(t, banned)
+}