I tried to do the memory bandwidth math but ended up even more puzzled:

Anandtech Nexus 7 review says it has 5.3 GB/s of memory bandwidth. At 60 fps that would leave 88 MB per frame.

The 1280x800 screen has 3 megabytes worth of pixels at 24 bpp. That's 1/29 of the 88MB-per-frame. So how come the overdraw related slowdowns started appearing with just 4x overdraw?