I asked a question at the last user group regarding differences in libraries
such as what do I get from JQuery that I don't get from other libraries. I still
havn't really compared them but I did come across these performance metrics
between the various libraries for each major browser. So atleast you can get a
sense of performacnce trade of. And yes, each library can have significant
performance hit as well as the browser it is ran on.

http://dante.dojotoolkit.org/taskspeed/report/charts.html