It would be interesting to see some additional data - for instance, the load & perceived render times for each browser under different operating systems - how is Safari on OS X vs Safari on Windows? Chrome & Firefox across Linux, Mac, and Windows?

I imagine that Safari is predominantly on OS X, and IE on Windows. Does that affect the load and render times? I guess since the two fastest (Firefox & Chrome) are also on all platforms, and probably more equally distributed, that would suggest that the OS isn't as important - but without more information, that is just a guess.