I've been playing around with RealityKit (https://developer.apple.com/documentation/realitykit) lately, it's their SceneKit replacement (they deprecated it this year).

On tvOS if a frame takes too long it just starts skipping frames and runs at 20 FPS. It won't recover after it happens. ThermalState is .nominal, so it's not that. There's also no way to just say "render as fast as possible" so you can measure performance, instead every device does throttling/pacing and it's completely opaque.

Anyway, I've had a developer account for 10+ years, and run into stuff like this constantly where you just can't get things to work properly. The framework isn't used much, so it's probably just broken.

I ported the game to SDL3/Zig and wouldn't you know it I can run continuously at 60 fps.. It's disappointing because I'm on board with targeting a single platform if I can utilize the hardware more effectively and make something special with it. But instead it's constantly an uphill battle of dealing with the opaqueness/brokenness of the platform. If I open a feedback request I'll probably just get told it's a bug, and maybe they fix it next WWDC.