The problem at Chaengwattana is the queue. I usually try to get there before 7:30 so I can get a good queue (in the 150-200 range). Officers line you up making sure nobody is skipping the line. Then at 8:30 they open the doors and you get in to reach another counter where they will give you an actual queue ticket (if you have all documents in order). If you lose time filling the form (available only after they open the doors) before getting the queue then scores of people will come before you. I ususaly manage to be out by 9:30 am but that is only because i come early with all documents in hand (NB queue for extension is not as bad as the others)