Decode large glTF-binary JSON chunks off the main thread

ReadToEndAsync completes synchronously on a memory stream, stalling the
main thread ~100 ms on a 12.6 MB JSON chunk. Gate the decode through the
defer agent like the parse already is.

Author: nickdnk

Packages/com.unity.cloud.gltfast/CHANGELOG.md

@@ -6,6 +6,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Main thread stall when loading glTF-binary files with large JSON chunks: the JSON string decode now runs on a background thread (gated by the defer agent, like the JSON parse), instead of always executing synchronously on the calling thread.
+
+## [6.19.0] - 2026-05-19
+
 ### Added
 - (Add-Ons) Import glTF animations to custom animation systems.
   - [IAnimationProcessor](xref:GLTFast.Animations.IAnimationProcessor) — animation clips conversion

Packages/com.unity.cloud.gltfast/Runtime/Scripts/GltfImport.cs

@@ -2075,11 +2075,38 @@ CancellationToken cancellationToken
                 {
                     Assert.IsNull(Root);
 
-                    Profiler.BeginSample("GetJSON");
                     var bytesStream = bytes.ToUnmanagedMemoryStream((uint)index, chLength);
                     var reader = new StreamReader(bytesStream);
-                    var json = await reader.ReadToEndAsync();
-                    Profiler.EndSample();
+                    string json = null;
+                    var predictedTime = chLength / (float)k_JsonParseSpeed;
+#if GLTFAST_THREADS && !MEASURE_TIMINGS
+                    if (DeferAgent.ShouldDefer(predictedTime))
+                    {
+                        // ReadToEndAsync completes synchronously on a memory stream
+                        // => decode in a thread.
+                        try
+                        {
+                            json = await Task.Run(() =>
+                            {
+                                Profiler.BeginSample("GetJSON");
+                                var result = reader.ReadToEnd();
+                                Profiler.EndSample();
+                                return result;
+                            }, cancellationToken);
+                        }
+                        catch (OperationCanceledException)
+                        {
+                            cancellationToken.ThrowIfCancellationRequestedWithTracking();
+                        }
+                    }
+                    else
+#endif
+                    {
+                        // Chunk is small enough to decode within the frame budget.
+                        Profiler.BeginSample("GetJSON");
+                        json = await reader.ReadToEndAsync();
+                        Profiler.EndSample();
+                    }
 
                     var success = await ParseJsonAndLoadBuffers(json, cancellationToken);