[0x0]

You should consider switching to window.performance.now() as that is a monotonically increasing clock, instead of a wall clock that might randomly be adjusted with time sync services.

Also, maybe you should run the tests in sequence instead of at the same time. I wonder if running two concurrent fetches might disadvantage the second fetch?

[PNWChris]

Super great points, I'm going to experiment with these ideas a bit and see how it goes.