Pixel perfect layouts had NOTHING to do with that performance gap, and iOS devices have far and away the best GPUs of each generation. iOS devices consistenly push fewer pixels (lower resolution screens) with faster GPUs than Android does.
The only reason people think Android devices are such power houses is because everyone still just looks at the CPU core count and clock speed. Which has almost no impact on scrolling and animation performance.
EDIT: For example compare the Galaxy Nexus to the iPhone 4S. Both shipped Nov 2011, both use PowerVR GPUs. The GN is rendering 920k pixels, the 4S is only 615k pixels! The GN is making due with the SGX540 at 307MHz, whereas the 4S has the SGX543MP2 at 266mhz or so - literally double the GPU power of the GN, and for a lower resolution screen.
But this is no longer true, or at least true to a much lesser extent. With current hardware you can achieve 60fps reliably even with a layout engine running constraint evaluations for every frame. It makes sense to make it easier to target variable screen geometries now, which was not the case 7 years ago.