The trivial arithmetic operations (add, madd, div) in the Web Array Math API benchmark are extremely slow.
For example, a core loop that looks like this (x, y, dst are Float32Arrays):
for (var k = Math.min(dst.length, x.length, y.length) - 1; k >= 0; --k)
dst[k] = x[k] + y[k];
...is about 10x slower than a similar loop that contains Math.sqrt() instead of addition.
I suspect that the arithmetic operation or typed array accesses are slow-cased for some reason, or maybe the loop does not get any JIT love at all?