diff --git a/mobile/lib/shared/relay/media_auth.dart b/mobile/lib/shared/relay/media_auth.dart
index 8611ce5b56..cc423a99c6 100644
--- a/mobile/lib/shared/relay/media_auth.dart
+++ b/mobile/lib/shared/relay/media_auth.dart
@@ -31,6 +31,7 @@ const _mediaGetAuthRefreshMarginSeconds = 60;
 class MediaGetAuthService {
   final String _baseUrl;
   final String? _nsec;
+  final String? _pubkey;
   final DateTime Function() _now;
 
   Map<String, String>? _cachedHeaders;
@@ -39,9 +40,11 @@ class MediaGetAuthService {
   MediaGetAuthService({
     required String baseUrl,
     required String? nsec,
+    String? pubkey,
     DateTime Function()? now,
   }) : _baseUrl = baseUrl,
        _nsec = nsec,
+       _pubkey = pubkey,
        _now = now ?? DateTime.now;
 
   bool isRelayMediaUrl(String url) {
@@ -122,6 +125,8 @@ class MediaGetAuthService {
       content: 'Get buzz-media',
       tags: tags,
       secretKey: privkeyHex,
+      // Deriving the pubkey costs about a third of a signature, per image.
+      pubkey: _pubkey,
       verify: false,
     );
   }
@@ -129,7 +134,11 @@ class MediaGetAuthService {
 
 final mediaGetAuthServiceProvider = Provider<MediaGetAuthService>((ref) {
   final config = ref.watch(relayConfigProvider);
-  return MediaGetAuthService(baseUrl: config.baseUrl, nsec: config.nsec);
+  return MediaGetAuthService(
+    baseUrl: config.baseUrl,
+    nsec: config.nsec,
+    pubkey: ref.watch(myPubkeyProvider),
+  );
 });
 
 Map<String, String> mediaGetHeadersFor(WidgetRef ref, String url) {
diff --git a/mobile/lib/shared/relay/relay_session.dart b/mobile/lib/shared/relay/relay_session.dart
index 18e03bb2b7..3173afc85f 100644
--- a/mobile/lib/shared/relay/relay_session.dart
+++ b/mobile/lib/shared/relay/relay_session.dart
@@ -245,6 +245,7 @@ class RelaySessionNotifier extends Notifier<SessionState> {
           url: url,
           bodyBytes: bodyBytes,
           nsec: config.nsec,
+          pubkey: ref.read(myPubkeyProvider),
         ),
         'Content-Type': 'application/json',
       },
diff --git a/mobile/lib/shared/relay/relay_session_auth.dart b/mobile/lib/shared/relay/relay_session_auth.dart
index 9c6c79ae02..a198fc0151 100644
--- a/mobile/lib/shared/relay/relay_session_auth.dart
+++ b/mobile/lib/shared/relay/relay_session_auth.dart
@@ -1,10 +1,13 @@
 part of 'relay_session.dart';
 
+/// Pass the signer's hex [pubkey] when it is already known: deriving it from
+/// the key costs about a third of a signature, on the UI isolate.
 String buildNip98AuthHeader({
   required String method,
   required String url,
   required List<int> bodyBytes,
   required String? nsec,
+  String? pubkey,
 }) {
   if (nsec == null || nsec.isEmpty) {
     throw Exception('Cannot query relay: no signing key available');
@@ -27,6 +30,7 @@ String buildNip98AuthHeader({
       ['nonce', const Uuid().v4()],
     ],
     secretKey: privkeyHex,
+    pubkey: pubkey,
     verify: false,
   );
   return 'Nostr ${base64.encode(utf8.encode(event.toJson()))}';