the indexmenu topic on the dokuwiki manual pages shedded some light on my problem, so for information purposes i'm posting it here aswell. (
http://wiki.splitbrain.org/plugin:indexmenu#bugs)
"Unfortunatly indexmenu has always to retrive the informations of all sub-pages, this is why it’s slow with a lot of files when cache expires. Max option speeds up only the client browser because it’s related to javascript rendering. I’ll look for an ajax solution, but consider also the good indexmenu2 plugin that it should do what you need. — Samuele Tognini 2007-06-11 17:39 "