Why does my Burst-compiled job run slowly on iOS?

AOT (Ahead Of Time) compiled Burst output for iOS missed the build, so jobs fall back to managed Mono execution. Confirm Burst is enabled, target platform is included, and the AOT step finished without errors.

Where do AOT libraries land?

Library/Burst/AOT/ /. After build, they're packaged into the player. Editor logs print 'Burst AOT compilation done' on success.

How do I confirm Burst ran on device?

Add a development build, attach Profiler. Burst-compiled jobs show with a blue 'Burst' badge in the Job hierarchy. If they show without it, you're falling back to managed.

Fix: Unity Burst AOT Library Stripped from Cross-Platform Build

Quick answer: Project Settings → Burst AOT Settings: tick the target platform. Verify the build log includes “Burst AOT compilation done.” Use a development Player + Profiler to confirm jobs show the Burst badge on device.

Burst job runs at expected speed on PC. iOS build runs the same job 10x slower. The AOT path didn’t produce a Burst library for iOS.

The Symptom

Performance regression on a specific platform compared to Editor / desktop. Profiler doesn’t show the Burst badge on jobs. No errors at runtime.

The Fix

Step 1: Burst AOT Settings. Project Settings → Burst AOT Settings. Per-platform: tick Enable Burst Compilation. Default Enabled = Standalone; you must add iOS, Android, etc.

Step 2: Build with Burst. Build & Run. Watch the editor log for:

Burst: Compiled 42 jobs in 8.3s for player target StandaloneOSX
Burst: AOT compilation done.

If the log mentions errors, fix those first. Common causes: managed allocations in jobs, unsupported instructions on the target.

Step 3: Confirm on device. Development build with Auto-Connect Profiler. Jobs view shows blue Burst badge per job.

Native Iterations Per Frame

Use the Burst Inspector (Window → Burst → Open Inspector) to view generated assembly per platform. Confirm SIMD vectorization happened. If Burst-compiled but not vectorized, you may have a managed reference forcing a slow path.

Verifying

Profile the same job on PC and on iOS. Per-iteration time should be similar (within 2–3x for ARM vs x86). Larger gap means iOS isn’t Bursted.

“Tick the platform in AOT Settings. Build. Confirm via Profiler. Burst on every target.”

Related Issues

For Burst BC1006, see Burst BC1006. For data race, see data race.

Tick. Build. Confirm. Burst on every target.