Well, here again NSK wrote:
Wikipedia/WikiCommons/Wikibooks are all dangerously slow. I recently installed the Zend Optimiser 2.5.7 on my site www.wikinerds.org . Zend Optimiser speeds up php by 40%. I think you should install it too if you haven't done so already.
Have you tried Turck? It's on all pages you may find about MediaWiki performance tuning. http://turck-mmcache.sourceforge.net/. And yes, wikimedia sites sure run bytecode cache. You may try other caching facilities (memcached, squid, ...) as well, then you could tell that wikineds.org is running xxx% faster ;-)
Domas