I've used Preswick recently, and maybe I was lucky, but there were very small queues, and I was checked in within about 10 minutes.
I seem to remember that they used to advertise that if your check-in took more than so many minutes, you would be automatically given compensation.
The terminal is of course old fashioned, but it still does the job reasonably well.
The comment about the reason other operators are not using Prestwick due to the state of the Terminal simply doesn't hold water! Airlines operate on a purely commercial basis, and if an Airline were depending on connecting passengers for much of it's business, it would, of course, use Glasgow or Edinburgh.
I personally operate out of Glasgow, and have no personal axe to grind on this subject