Mostly make sure your hard drive is defragged and that your database is regularly vacuuming itself in file->options->maintenance and processing. Under speed and memory of the same dialog, you can increase the threshold and timings that the autocomplete waits for before fetching results, including converting it to a completely manual system.
Autocomplete results are too slow right now for most users, no matter the size of their collections, so I would like to convert it to a streaming system. Actually matching your search input to the list of tags only takes about 0.15s, but gathering accurate file counts for a specific search domain is what takes so long, so I want to see about chopping that stage up into smaller bits, especially since the vast majority of your search results are off screen and don't need accurate results quickly if I can roughly sort them otherwise.
I will have this big rewrite on the soon-to-come voting list of new features.