diff --git a/mobile/lib/shared/utils/string_utils.dart b/mobile/lib/shared/utils/string_utils.dart
index df9fa2c238..177a4ec6aa 100644
--- a/mobile/lib/shared/utils/string_utils.dart
+++ b/mobile/lib/shared/utils/string_utils.dart
@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart' show visibleForTesting;
 import 'package:nostr/nostr.dart' as nostr;
 
 /// Neutral label rendered when an identity string cannot be read as a Nostr
@@ -49,8 +50,24 @@ String? fullNpub(String identity) {
   }
 }
 
-String _encodeNpub(String hexPubkey) =>
-    nostr.Nip19.encode(prefix: nostr.Nip19Prefix.npub, data: hexPubkey);
+/// Encoded npubs by lowercase hex key. Identity views label every roster
+/// member without a profile name by npub on each rebuild, and bech32 encoding
+/// dominated those rebuilds on large channels. Bounded: once full it keeps
+/// what it has, so a roster larger than the limit still reuses most keys.
+final _npubs = <String, String>{};
+@visibleForTesting
+const npubCacheLimit = 8192;
+
+String _encodeNpub(String hexPubkey) {
+  final cached = _npubs[hexPubkey];
+  if (cached != null) return cached;
+  final npub = nostr.Nip19.encode(
+    prefix: nostr.Nip19Prefix.npub,
+    data: hexPubkey,
+  );
+  if (_npubs.length < npubCacheLimit) _npubs[hexPubkey] = npub;
+  return npub;
+}
 
 /// The canonical compact public-key label: the first 8 and last 4 characters
 /// of the full npub joined by an ellipsis — the same truncation desktop uses
diff --git a/mobile/test/shared/utils/string_utils_test.dart b/mobile/test/shared/utils/string_utils_test.dart
index d11754d52d..fc5e7f4430 100644
--- a/mobile/test/shared/utils/string_utils_test.dart
+++ b/mobile/test/shared/utils/string_utils_test.dart
@@ -16,6 +16,23 @@ void main() {
   const canonicalCompact = 'npub180c\u2026h6w6';
 
   group('fullNpub', () {
+    test('reuses the encoding of a key it has already encoded', () {
+      // Identity views relabel a large roster on every rebuild; each key's
+      // bech32 encoding must happen once, not once per rebuild.
+      final first = fullNpub(canonicalHex);
+      expect(first, canonicalNpub);
+      expect(identical(fullNpub(canonicalHex.toUpperCase()), first), isTrue);
+    });
+
+    test('a roster larger than the cache still reuses its encodings', () {
+      String key(int i) => i.toRadixString(16).padLeft(64, '0');
+      final firstPass = [
+        for (var i = 0; i <= npubCacheLimit; i++) fullNpub(key(i)),
+      ];
+      // A second scan in the same order finds the early keys still cached.
+      expect(identical(fullNpub(key(1)), firstPass[1]), isTrue);
+    });
+
     test('canonicalizes every accepted input wrapper', () {
       // Every accepted wrapper — lowercase hex, uppercase hex, whitespace-
       // padded, and an already-npub key — converges on the canonical npub.