Ah, I think I may have found them problem. About 4000 of the 10000 images had 'nude' in thier title. D'oh! I've nixed the Boticelli and Modigliani collections along with a few others to see if this improves matters.

Failing that, I might try digging out a java profiler to see if there's a particular bottleneck. Are the developers open to feedback of this sort ?