Regarding the "power" issue: iPads today are 1.x (1.4?) GHz Dual Core computing beasts with graphics coprocessor performance that's just amazing.

My NeXT Cube 2nd gen had a 25 MHz 68040, so even giving some slack that's a factor 40-50 slower, and it was not just OK as a content-creation machine, it was awesome.

When software is slow, it is usually the fault of the software, not a limitation of the hardware.

That doesn't mean that other cases don't exist, and rendering certainly sounds like one of them, but they are rare, and may then be better handled by remote server farms anyway.