I had a 2.5 hour connection in ORD, inbound on what I think was that same UA flight from LHR. I made my onward flight by a hair, was the last or 2nd to last passenger onto the plane.

The walk to the immigration line, and immigration line itself, was about 60 minutes, bags arrived 15 minutes after I got out of that line, and then add another 15 mins to security via the train, and another staggering 45 minutes in the premier access security line (! -- right during rush hour) and a 5 minute dash to my gate, and I was there (barely).

The next time I came into ORD on the same flight immigration was all of 3 minutes, and because this time I didn't check bags I was at the UA terminal 15 minutes later.

Bizarre. One thing is certain -- despite owning like half of ORD, almost all of the UA flights get parked all the way at the far end of the terminal so you get to walk the 15 minutes.