Derek Haynes laments:

Auto-Complete is a great tool when it provides possible results BEFORE you finish typing. Unfortunately, using Rails’s included AJAX helpers to query the database as you type often results in a large delay before matches are returned.

There have been small hacks from several people to help resolve this, but Derek presents a simple local_auto_complete_field helper (with source to use right away) that lets you specify some pre-fetched results for the autocompleter to use right away.