There's already a bit delay built in.. for me fine, but maybe for reducing load on busy server an optional extra delay will be welcome.
Maybe for busy servers it's usefull too to limit the number of returned matches. (i got the idea that now all matches are showed)
My test case: http://www.dokuwiki.org/start?do=search&id=plu
An annoyance for me is that the suggestions disappear when you type again in the search field. So when you were looking on the suggestions during typing, these were vanished. Not so helpfull.
(This jquery plugin has some of these properties by using some caching https://github.com/agarzola/jQueryAutocompletePlugin/wiki
- as example to illustrate the idea)