I came across an interesting performance issue using #urlget (from the URLGetparameters extension).
The details are here : www.mediawiki.org/wiki/Extension_talk:UrlGetParameters#Performance
I know it is not directly related to SMW but since I used this extension to customize calls to semantic queries based on URL parameters, this may be relevant to other people doing the same thing.
I was able to reduce access times from 15 seconds to 3 seconds on a page using this tip.