In my experience, with typical OS navigations (scrolling lists, screens, etc), all three generations of iPads (and all generations of iPhones) have been very smooth. Android devices have come a long way in a few short years. Many phones are now just as smooth as iDevices, but here and there, you can still feel some stutter. I'm fairly convinced that Project Butter will address this. It's been a known issue that screen refresh CPU priority in Android was not high and they finally decided to address it.
I've yet to hear of anyone complaining that the Nexus 7 tablet is choppy in any way.
The good thing about science is that it's true whether or not you believe in it. -Neil deGrasse Tyson
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