I have noticed that JQuery UI is particularly slow in Firefox. This affects me both when using the code on my site and when viewing their own page. It dawned on me recently that the cause of this was the fact that I tend to leave firebug on.
Secondly, I've noticed that when possible it is a heck of a lot faster to use the browser's native sliding feature (by say turning scrollbars on in a div using overflow=auto or overflow=scroll and setting a hard width) than using JQuery UI's sliding.
Thirdly, because of the overhead of the JQuery UI I have found it faster in many cases just to write my own logic that more specifically fits the needs of the application at hand. Using the UI to get some ideas, but then applying my own logic to the problem at hand.
So for a re-cap of how to speed up building UI tools with JQuery:
- Disable Firebug (and encourage your visitors to)
- Use Native Scrolling when Possible
- Write your own code
JQuery UI: http://www.jqueryui.com/