It started loading faster. I don't know, maybe it was preparing the db. But, still, this approach of loading all users into the pull down list will become a problem when the user base grows? - yes, of course. If you have lots of users that form with dropdown is going to take much time to load.
You can either use model caching or implement a bit more advanced solution with AJAX to lazy load only the records that match the entered sample. Also it is a nice idea to send AJAX request when at least two (or three maybe) letters are entered in the field. You may also employ a jQuery plugin that fires an event when user stopped typing (or create such plugin yourself as it shouldn’t be hard).