For math calculations amenable to SIMD, browser vendors have already designed in a huge advantage for WebAssembly: plans for SIMD support in JS was dropped, and kept for WebAssembly. At the start it gives up to a 4x advantage to WebAssembly with the basic 128-bit SIMD, but possibly 16x later if/when WebAssembly gets support for vector widths corresponding to today's hardware.

(anyone know why they started with narrow SIMD? surely it's easier to emulate 512-bit simd with 128-bit simd instructions than try to compile 128-bit simd to take advantage of 512-bit simd?)