Beginning of February, my flight was scheduled to land 6am but it was a bit early. I had business ticket so I got out of the plane fast, but didn't even bother to go to fast track immigration since the normal was completely empty.
I walked to the counter, 1 minute checking documents, stamp stamp and I was out.
Total time from exiting plane to airport train was like 20 minutes.

today late morning not very busy in arrival hall east
at the reopened special fast track between arrival hall east and west, waiting time zero...

Yesterday (Monday) lunchtime came from Concourse G and noticed screens showing immigration wait time of 3 minutes. Passed by first regular immigration ramp and there wasn't much activity so screens could well have been accurate.

Fast track closed and directed to a new area to the right of the second immigration hall. Looked temporary with makeshift signs but 4 agents on duty and no queues.

I arrived about a week ago (March 24) at the "busy" early AM arrival bank from Europe - 6AM hour.
Hardly any line at all and through in about 5 minutes.

Arrived BKK 0500 from HND with need to collect bag and check in with Thai Smile to HKT. I had access to Fast Track using C borading Pass. All in it took about 45 minutes from gate (1Km) to Immigration to Baggage Clain/Customs to check-in at Thai Smile. Important note: I was traveling in Premium Economy (U booked through Thai Airways website using voucher from 2019) and that line was only a 5 minute que to desk (1-2 people ahead of me).

Cheers,

-Cyborg

Arrived 30 April from IST around 15:00 and since flying business could use fast track - just 1 person before me at immigration so through in minutes. The walking from the gate added another 10-15 minutes. Did not check queues for normal immigration in detail but I did not have the impression they were overly long.

Arrived around 9 PM - zero people in line for immigration after getting off my flight in mid-economy. I almost went to get a visa on arrival before realizing that was not needed for US tourists - just walk to Immigration.

Arrived on 16 May during the 3pm bank of flights. Lucky enough, we got off at D6 - right at the entrance to immigration. Saw a HUGE wave of a few hundred people charging toward immigration, as I made the turn past VOA to fast track. I assume some folks got caught in a massive line.

Ofc, inside fast track no line was longer than 1 person deep. It also turned out that my baggage claim was directly in front of the exit from fast track.

You should buy a lotto ticket.

I arrive BKK in a week, 2:30 in the afternoon on a Monday. I'll have fast track so from people's comments it sounds like its a pretty quick experience. I did not notice in this thread any comments on paperwork. Is there a form and anything to do online for declarations?

I do not think so when I traveled there last month. They do ask for your boarding pass at immigration. They did away with the arrival card.

Nothing to fill out anymore - just make sure you fulfil the entry criteria for your nationality, and you will be fine. As others have mentioned they will ask for your boarding pass though. Enjoy your trip :)