a Debian user requested a feature for tumbler: that it could use multiple CPU cores to do thumbnailing. That'd be nice since more and more multi-core CPU exist (even on smartphones :) and parallelizing thumbnail generation would help doing that faster.
This is already implemented but for some reason that I don't remember, the schedulers in tumblerd only use one thread at the moment (as can be seen here: http://git.xfce.org/xfce/tumbler/tree/tumblerd/tumbler-lifo-scheduler.c#n141).
I'm not sure we want to increase this as it might as well be a good idea not to waste all CPUs on generating thumbnails. Other applications might benefit from the free CPU(s).
It'd be a little more complicated but maybe check the current load and calcul the optimal number of threads according to the load and the number of cores? The faster it's done, the faster the cpu is available for doing something else (as long as the UI is still responsive, obviously :)
*** This bug has been marked as a duplicate of bug 3703 ***