diff --git a/lib/providers/navigation_geometry_provider.dart b/lib/providers/navigation_geometry_provider.dart
index 1d469d15..d1f50ca7 100644
--- a/lib/providers/navigation_geometry_provider.dart
+++ b/lib/providers/navigation_geometry_provider.dart
@@ -56,13 +56,13 @@ Future<NavigationGeometryMap> loadNavigationGeometry(MapValue map,
   ));
 }
 
-NavigationGeometryMap _decodeNavigation(
+Future<NavigationGeometryMap> _decodeNavigation(
     ({
       MapValue map,
       Uint8List bytes,
       Offset defenseOffset,
-    }) source) {
-  final decoded = jsonDecode(utf8.decode(decodeWorldGzip(source.bytes)));
+    }) source) async {
+  final decoded = jsonDecode(utf8.decode(await inflateWorldGzip(source.bytes)));
   if (decoded is! Map<String, dynamic> || decoded['map'] != source.map.name) {
     throw const FormatException('Navigation geometry map mismatch.');
   }
diff --git a/lib/providers/svg_height_runtime_provider.dart b/lib/providers/svg_height_runtime_provider.dart
index fd05a21f..77c3461e 100644
--- a/lib/providers/svg_height_runtime_provider.dart
+++ b/lib/providers/svg_height_runtime_provider.dart
@@ -258,8 +258,8 @@ Future<SvgHeightVisibility> _loadSide(Uint8List source, List<int> artworkBytes,
   return model;
 }
 
-Map<String, dynamic> _decodeModel(Uint8List source) {
-  final decoded = jsonDecode(utf8.decode(decodeWorldGzip(source)));
+Future<Map<String, dynamic>> _decodeModel(Uint8List source) async {
+  final decoded = jsonDecode(utf8.decode(await inflateWorldGzip(source)));
   if (decoded is! Map<String, dynamic>) {
     throw const FormatException('SVG sightline asset must be an object.');
   }
diff --git a/lib/view_cone/vision_world_gzip.dart b/lib/view_cone/vision_world_gzip.dart
index 863c5809..a2dead26 100644
--- a/lib/view_cone/vision_world_gzip.dart
+++ b/lib/view_cone/vision_world_gzip.dart
@@ -2,14 +2,13 @@ import 'dart:typed_data';
 
 import 'package:archive/archive.dart';
 
+import 'vision_world_gzip_web.dart'
+    if (dart.library.io) 'vision_world_gzip_io.dart' as platform;
+
 /// archive uses native zlib on desktop and its Dart decoder on web. The SDK
 /// accepts partial streams, so untrusted blocks also need their complete footer.
 Uint8List decodeWorldGzip(Uint8List compressed, {bool verifyChecksum = true}) {
-  if (compressed.length < 18 ||
-      compressed[0] != 0x1f ||
-      compressed[1] != 0x8b) {
-    throw const FormatException('Invalid world gzip header.');
-  }
+  checkWorldGzipHeader(compressed);
   final decoded = const GZipDecoder().decodeBytes(compressed, verify: true);
   final footer = ByteData.sublistView(compressed, compressed.length - 8);
   if (footer.getUint32(4, Endian.little) != decoded.length ||
@@ -19,3 +18,17 @@ Uint8List decodeWorldGzip(Uint8List compressed, {bool verifyChecksum = true}) {
   }
   return decoded;
 }
+
+/// [decodeWorldGzip] with the same checks. On the web the browser's own gzip
+/// inflates and verifies the data: there the Dart decoder and CRC run on the
+/// UI thread and take about 0.4 s per side of a large map.
+Future<Uint8List> inflateWorldGzip(Uint8List compressed) =>
+    platform.inflateWorldGzip(compressed);
+
+void checkWorldGzipHeader(Uint8List compressed) {
+  if (compressed.length < 18 ||
+      compressed[0] != 0x1f ||
+      compressed[1] != 0x8b) {
+    throw const FormatException('Invalid world gzip header.');
+  }
+}
diff --git a/lib/view_cone/vision_world_gzip_io.dart b/lib/view_cone/vision_world_gzip_io.dart
new file mode 100644
index 00000000..fdec57f6
--- /dev/null
+++ b/lib/view_cone/vision_world_gzip_io.dart
@@ -0,0 +1,7 @@
+import 'dart:typed_data';
+
+import 'vision_world_gzip.dart';
+
+/// Desktop's zlib is native and fast; it runs in a background isolate.
+Future<Uint8List> inflateWorldGzip(Uint8List compressed) async =>
+    decodeWorldGzip(compressed);
diff --git a/lib/view_cone/vision_world_gzip_web.dart b/lib/view_cone/vision_world_gzip_web.dart
new file mode 100644
index 00000000..62ebc8aa
--- /dev/null
+++ b/lib/view_cone/vision_world_gzip_web.dart
@@ -0,0 +1,47 @@
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+import 'dart:typed_data';
+
+import 'vision_world_gzip.dart';
+
+@JS('DecompressionStream')
+extension type _DecompressionStream._(JSObject _) implements JSObject {
+  external factory _DecompressionStream(String format);
+}
+
+extension type _ReadableStream._(JSObject _) implements JSObject {
+  external _ReadableStream pipeThrough(JSObject transform);
+}
+
+@JS('Response')
+extension type _Response._(JSObject _) implements JSObject {
+  external factory _Response(JSAny body);
+  external _ReadableStream get body;
+  external JSPromise<JSArrayBuffer> arrayBuffer();
+}
+
+/// The browser's gzip checks what [decodeWorldGzip] checks: it rejects a
+/// wrong CRC-32 or length, a stream that ends before its footer, and bytes
+/// after it. The length is compared again here so a short read cannot pass.
+Future<Uint8List> inflateWorldGzip(Uint8List compressed) async {
+  // Firefox before 113 and Safari before 16.4 lack it; they keep the slow path.
+  if (!globalContext.has('DecompressionStream')) {
+    return decodeWorldGzip(compressed);
+  }
+  checkWorldGzipHeader(compressed);
+  final Uint8List decoded;
+  try {
+    final inflated = _Response(compressed.toJS)
+        .body
+        .pipeThrough(_DecompressionStream('gzip'));
+    decoded =
+        (await _Response(inflated).arrayBuffer().toDart).toDart.asUint8List();
+  } on Object {
+    throw const FormatException('Invalid world gzip checksum or length.');
+  }
+  final footer = ByteData.sublistView(compressed, compressed.length - 8);
+  if (footer.getUint32(4, Endian.little) != decoded.length) {
+    throw const FormatException('Invalid world gzip checksum or length.');
+  }
+  return decoded;
+}