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 split in 2: destroy, option string generation, dom node insert; then the actual initialization of the new UFD. The latter is the relevant part, as the first is just generating dummy data!
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.