There are alot of speed tests on the web that look at javascript libraries like jQuery.  All of them have been getting faster and faster.  I used a MooTools template, which was actually pretty easy to set up.  I pointed it at my blog rather than the typical Shakespeare script.  I also included the size of the libraries, which I think is important.

One thing I noticed is that there are ALOT of discrepancies.   The number of found items varies greatly.  MooTools especially fails on a few common selectors.  I was actually pretty liberal in downloading their library.  I got everything except the plugins.  Not sure what is going on there.

Additionally, I think its much more apparent using a real website how little these speed differences can be noticed by the human eye.  Much more importantly should be the accuracy of the findings.

See the test here: http://www.commadot.com/jquery/speedtest/