Hey all! Thought I would add my experiences to the knowledge here:
Interview on arrival at O'Hare ~Noon on a sunday. Asked at immigration (sent there, regular US passport) was directed to an are to the far left (as looking "across the border").

There was some signage and ~10 people ahead of me in line. For most of the time there was one agent processing, towards the end a second logged on and started working.

It took ~1hr of that ~5 mins was the actual interview. So many stressed folks in line trying to fit it in with a tight connection. When I left line was double in length..

Take away: If you plan to enroll on entry there is a chance it could take an hour at O'Hare.. plan your flights accordingly

Got the approved email ~3 hours later

Oh an addendum: You will need your PASSID.