I just compared the song generation time in the JavaScript port of Sonant to the hand optimized x86 assembler version of the same synth, as used in the original Muon Baryon 4k demo.
I found the SNT tune here, and ran it in Opera in my new online Sonant player (check “Play Now!” if you wish to play the song in your browser), and guess what:
Native asm synth: 43s
JavaScript synth: 47s
That means that the JavaScript synth is less than 10% slower than the hand optimized native assembly language synth!