The state of SIMD.js performance in Firefox
blog.mozilla.org
blog.mozilla.org
Given that SIMD should bring about a 4 times speedup and yet the polyfill is 17 times slower than the Ion SIMD version and 40 times slower than the asm.js SIMD version, it seem likely that an implementation of the same algorithm without SIMD or polyfill would vastly outperform the polyfill too.
a[0] *= b[0];
a[1] *= b[1];
a[2] *= b[2];
a[3] *= b[3];