These tests aren't particularly meaningful. They're usually more dependent on network speed. As far as actual rendering only the Android phone fared poorly because it wasn't smooth.
On my iPhone4 I tried Amazon and Gizmodo and could only get checkerboards when scrolling quickly down and even then they were brief flashes and not the longer times of checkerboard city I was getting with my old 3G. I'm guessing it's a memory management thing.
But more than anything, the WP7 browser UI seemed just awful. I hope they seriously improve it because having things like Windows style checkboxes and radio buttons on a phone is just poor UI design. The tab management didn't look good either. The thumbnails were tiny even with just one tab open and the close buttons were huge. I imagine you'd end up accidentally closing tabs a lot.
Another thing I worry about is rendering engine features. Is it based on IE9 tech or some old crap that won't support CSS3 or any other modern web features?
Now that the top smartphones are all roughly on the same level as far as performance goes, it comes down to usability. So far Apple is still the leader in that area IMO.
