[mattzito]

Accuracy aside, which I'm sure can be improved to a certain degree, I don't think this is a desirable property for a browser to expose to javascript. Just because a node has a certain number of cores doesn't mean they're available to javascript, or that they even represent a realistic picture of what the machine has.

As other people have pointed out, things like Intel speed boost change the performance characteristics of the machine, and things like virtualization can flat-out lie about the capacity of the machine.

It seems like a far more preferable solution is to lightly parallelize based on generic defaults and let the OS handle switching, instead of trying to outsmart the system.