The popular external encoder libraries such as x264/x265/libvpx/libaom-av1 as well as a lot of parts of ffmpeg itself have fine-tuned assembly and sometimes C f

The popular external encoder libraries such as x264/x265/libvpx/libaom-av1 as well as a lot of parts of ffmpeg itself have fine-tuned assembly and sometimes C for a lot of things that you'd be missing out on. So I'd guess the speed is more than a bit slower, at least when run on common x84-64 and arm targets, even considering any WASI runtime JIT.