EDIT (2013): try Boomerang instead, like @yasei-no-umi suggests. It's actively maintained.
-- old answer --
We use Jiffy.
It's very simple to use and modify - we wrote our own server-side code (instead of Jiffy's Apache + perl) and used Jiffy's JS.
Regarding a performance penalty - there isn't one on the client side. The JS itself is trivially fast, and the reporting back to the server can be done with an XHR after page load, which affects nothing in the user experience. On the server side you'll get twice as many requests. If that's a bottleneck, you could set up a different server just for the timing responses.