Well I generally use Mozilla. Its the browser installed on my main office machine which is a lowly 450MHz. I installed a MOOX firefox build on my home XP2600.
I noted no difference between the 2600 and the 450. In fact running the CPU% performance monitor on the 450 it is pretty rare that CPU usage ever gets anywhere near 70%.
I do have a nice disk setup on the 450.
So firefox, as I see it, is not bound by the CPU. Rather its limited by disk access (especially & suprisingly - disk writes). What's the point of optimizing the builds for flavors of ISA then?