Checked through my emails and when I connected there last year I had 2.5 hours. It was loads of time, allowing for time in the Delta SkyClub and taking my time. I agree with wanting to have 2 though. Outbound from UK could be delayed, you have to collect your luggage and drop it off at the transfer, go through TSA again, and more than likely need to get the Plane Train to another terminal (I had enough time to potentially walk, but didn't bother). Could pick up at a delay at any of those points, and imagine it could be stressful ++