As a heads-up, I've attacked the problem for GNU Emacs by rejiggering its makefile so that the info files can be built in parallel. This brings the total time for "make info" on my 4-core host down from 224 seconds to 86 seconds. Still pretty slow compared to the old 6.5 seconds, I'm afraid. The main bottleneck is that a couple of the Emacs manuals are large and their info files can't be parallelized.
Here's the Emacs patch that parallelized the doc builds: http://bzr.savannah.gnu.org/lh/emacs/trunk/revision/111850 I'm writing this not to complain (your efforts in improving Texinfo are appreciated) but just to help motivate any performance-improving techniques that might be coming down the pike.
