Should it? Both compile to intermediate representations which are distributed, then compiled to machine code by the browser. I see no reason PNaCl should be any faster.

Well you have benchmarks in that talk - PNaCl is 1.2x native speed. asm.js is ~2x. Single threaded. With multiple threads PNaCl will smoke asm.js since the latter has no threads at all.

A 2x performance difference means your phone lasts 3 hours instead of 1.5 hours when playing that game. This is a huge, huge difference. Battery-powered devices make performance extremely important.

NaCL also supports pthreads.