I fly in and out quite often and I dread outbound immigration - has *always* been a killer, and now with renovations, they're a full section short. It's truly annoying to see the Thai Passport queues empty and Foreign full and they don't even bother to ask you to step forward.
I fly with my Thai PA often, and on some occasions, they'll allow me to go with her through Thai Passport Holders, and at other times, I'll be waved back to the Foreign queue just as we are about to step forward, which is really bad especially when that line has gotten 3x longer! (Inbound they seem more agreeable to allow me into the country with her, go figure.)
Inbound can be long queues, but usually they're pretty good for the flights I'm returning on. I think they could improve, by allowing foreigners through their Thai lanes, when nobody is there, but that would mean thinking a little laterally - something I've found they don't do well at the best of times, so being a government department, it'll probably *never* happen any time soon.
I left yesterday for HKG, and it was pretty quick - the quickest I've been through in a long time - and that was through their newly renovated area.