Lots of thought has been put into initialization and filter speed for UFD. This demonstrates the initialisation time of UFD. Type in a number of items to generate, and click the button. The UFD will be destroyed, a new select node with random string items will be inserted, and a new UFD will be initialized.
Timing is only the actual initialization of the new UFD, and excludes random string and select construction times.
You can put any number, but keep it sane (<: 5,000) as initialization is proportional to list length, and CSS work on that many items isn't fast. In Firefox 3.5, one developer has noted his initialization is roughly: 20msec + (itemCount/4)msec.
There are a handful of other filtering implementations for you to compare.
Dijit.form.FilteringSelect is a comparable tool.