the tight loop iteration used a shared array buffer to provide a high resolution timer, specifically to deal with the obvious fix of truncating precision of performance.now().

The reason for /further/ truncating performance.now() is that the relative cost in this attack means that you don't need as much precision as was needed for the original (page table? I think) attack.

A SAB timer just needs to increment a counter in one thread and read it in the host thread and the granularity is however long it takes to get through a for-loop.