There shouldn't really be any performance difference between WebGL and WebGPU in a ShaderToy-like example, given that only requires a single draw-call per frame. The API overhead should be utterly negligible in that case.

If WebGPU really does perform better in an apples-to-apples comparison (i.e. running the same shader) then there's something seriously wrong with Apples WebGL implementation.

edit: here's that example translated verbatim to WebGL if you want to compare: https://www.shadertoy.com/view/Mt2fRy