The issue with this is that if you care about speed, you need to be using SIMD -- and wasm doesn't seem to want to support it -- stating a "minimum viable" standard from the year 2000.
For a dynamic language JIT this is pretty much infeasible (as I understand it). Every loop might have branches for guards/bailing out due to deopts, and at least in JS, TypedArrays are allowed to alias eachother.
I expect SIMD support can be added to wasm at some future date.
But, I was a little quick on the trigger. It's not obvious, but after drilling through the design docs for a half hour.. I found a tentative discussion of how SIMD might be added later (based on some kind extension mechanism).
To take an example where timeliness is crucial, think of the difference between, say, 10 frames per second vs. 60 frames per second in a first-person shooter game.