As I wrote in previous post, memcached didn't help as expected with wiki performace, so it was time to look for something else.
But today I decided it's time to try it. I built package from current version (you can get it on debian.cihar.com), installed and it seems to increase performance a lot. Most visible it is on wiki pages, which now take about quarter of time to process. I only hope it won't have any stability impacts, but it survived fine stress tests I did so far.