Icelandair uses gate A19. Everyone deplanes, walks a bit in a sterile corridor, and gets onto a Mobile Lounge for a ride to the CBP facility in the Terminal building. Try to stay close to the doors on the Mobile Lounge so you will be among the first off the lounge and into Immigration.

APC (Automated Passport Control) should be in place for US citizens by the end of this month, which should speed your processing even more. (Since you didn't mention it, I assume you don't have Global Entry - which would really speed the process.)