Thanks for providing that test.
So I quickly executed it on my platforms:

    Mac Mini      | Intel 4000      | Ubuntu 13.10 | Chrome 32 : 0.00 -> 0.02
    Mac Mini      | Intel 4000      | Ubuntu 13.10 | Firefox 25: 0.00 -> 0.02
    Dell Latitude | NVIDIA 9200M GS | Ubuntu 13.10 | Firefox 26: 0.01 -> 0.03
    Dell Latitude | NVIDIA 9200M GS | Ubuntu 13.10 | Chrome 31 : 0.01 -> 0.06

Let me see whether I can add OSX to that eventually.
Abviously `glFlush` is a non-issue for my systems but if it causes 1+ms delay on OSX, than I do agree that we should look into disabling it.
Still, I would like to run a few tests of my own and if they work fine without `glFlush`, I am ok with outcommenting the call, given that we document the derivation from docs.