You're right that a lot of the scaling probably happens with the GPU, and that Emscripten-generated code is still the most performant javascript possible (no GC, consistently typed for inference, etc).

I think there are places where the CPU is the bottleneck though, since we're talking about crazy complex 3d games. Managing a large scene graph, and updating physics are some examples. Simply reducing the viewable distance is a huge help when you are CPU bottlenecked for those reasons (most likely physics).

Agreed that no matter what, the Unreal port will help optimize the typed array/no GC code path! Exciting times.