I had recalled that if you're connecting to Chiang Mai, Phuket, Krabi, or Hat Yai you'd clear customs at your final destination.

I do see some references to clearing customs in Chiang Rai. I don't recall Chiang Rai on the "CIQ" signs when I've passed through BKK but that could just be my faulty memory. And I won't be passing through BKK until next month to check. Surely someone here must know for sure! I'll be flying into CEI on that trip as well, for the first time.

I'm truly ashamed of my ignorance, the fast track for pax other than TG F is new, so I haven't seen it reported whether this is available for transit immigration as well? Though I've never seen the transit lines particularly backed up personally.

My guess is that if your flight is on time, you don't have to claim bags, you're fine regardless -- and if either of those isn't the case then there's a potential challenge. But that fast track / escort / porter won't make the difference (but might reduce stress).

So the question is, is CEI treated as one of the airports where transit pax follow the CIQ signs to transit immigration?