I've heard quite a lot about Woopra, but also hear stories about their loadtime really slowing down your pages.
I run sites with a lot of graphical cntent and need fast loadtimes, as most of my uisers are quick moving websurfers, that will just leave a page if nothing shows within a second.

How does Woopra perform?

Seems to be no problem with me. Page loads pretty fast and if you place it a the bottom of your page all your content should be loaded before Woopra is called. Also once you go over 10.000 pageviews the app still keeps running, but just doesn't log your visits anymore.