That time is a very peak time. Sometimes immigration has been deserted if I arrive a bit early, and sometimes it take 45 minutes or more. There's a Vietnam Airlines flight that arrives about that time and it seems that they carefully screen all the Vietnamese passengers. So if you arrive before that flight it might not be too bad. If you arrive after it, it can take a very long time.

At that time I've only ever seen two immigration zones open. One for OZ and some of the A concourse passengers, and one for KE and I presume other A concourse passengers. I've never tried to walk over to the KE immigration zone, and don't know if there's anyone there to try to prevent you from doing so or not. Later in the day they do open up a second group of immigration counters next to the normal OZ one, and at that time it does often pay to check which one has shorter queues.

One suggestion for quicker processing. Stand in the rightmost foreigner queue, which should be adjacent to the leftmost Korean queue. If the flights arrive such that the Korean queues become empty, someone from immigration will direct some of the people from the rightmost foreigner queue to the Korean immigration counters. Normally this works out well for me, but once it was a big mistake and took me at least twice as long just because I guess the immigration officer assigned to my queue was extremely slow in processing passengers. Also look at how many passengers look like they may be Vietnamese or third-world South East Asians, Indians, Pakistanis, etc. If there's one line that doesn't have as many of these people, choose that line and it will normally be faster.