IMO Denver Immigration is a breeze because they have only 3 int. Flights a day(FRA+MUC,LHR).
After you dropped your luggage you have to go trouh security again, which take some time and afterwards there is a long walk(and train ride) to your UA Gate.
In september I had 2:30h conneting time between a LH Flight and a UAX Flight. I needed about 2h(beeing the first one out of the plane) and had 30min left in the RCC.