Afaik, Android does not make much use of the GPU for UI acceleration. That's most of the reason it seems laggier on slower hardware. They also made some significant performance improvements in 2.2, so hardware stuck on 2.1 or earlier also performs slower.

By contrast, WiPho7 and iOS use the GPU to accelerate the springboard and transitions. That's why the original iPhone and 3G were so smooth for only having a 400MHz CPU, and that's why all WiPho7 devices are required to use that Qualcomm SoC - it's the only currently supported GPU, and GPU is mandatory for the interface.