That is a tough hour to land in--a lot of flights coming in. PHL is my home airport and I can tell you that immigration there is a complete crapshoot--sometimes there is absolutely no one in the immigration hall and sometimes it's jammed. Still, with two hours you should be OK. The other thing about PHL is that baggage claim can be just as bad as immigration, so if you are checking bags, you'll probably end up waiting for either immigration or the bag anyway. Use the A-to-F shuttle bus once you get back airside.
I do have Global Entry and thank my stars for it. Last time I came in, I was on the early BA flight and it was a bit delayed, landing around 4 PM. The place was a zoo. I strolled right on past all those poor people standing in line and was out the door of the airport in 5 minutes.