Is there a technical restriction why JS JITs don't transparently apply SIMD to existing loops like normal compilers (=autovectorization)? Or is it in the cards to build it on top of SIMD.js?

I wasn't really referring to wasm vs JS (which is easy to outperform), but reaching equivalent speed to native, which is the ultimate goal, I'd hope. Otherwise you're just throwing performance away...

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).

SIMD isn’t the “end all”, but upcoming Intel chips have instructions for handling 8 doubles or 16 floats per instruction, and if you’re trying to implement a video codec or large-scale physics simulation, an order of magnitude of speed difference can make or break your app.

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.