Are tablet CPUs fast enough?

Probably not for a whole book, but for a single chapter? Almost certainly. And if you eschew the features of XeTeX or LuaTeX and just stick with pdfTeX, things could be very quick: For the example I posted elsewhere in this thread of typesetting a novel in 30s with XeLaTeX, pdfTeX does it in about 6s. A single chapter (32k of text): 0.8s, and I/O bound at that. Even if a tablet's 800Mhz Cortex A8 does that 5 times slower than my Core 2, it's fast enough to be done speculatively. It's certainly no slower than loading a web page over the internet.