Well I experienced the reverse of this at EWR last week. Landed 30 minutes early and got to the gate 45 minutes after scheduled arrival time. The EWR problem at least is that whoever was putting together the board for that day failed miserably at the sequencing of planes in and out of the gate area (including getting planes who landed near the far C gates over to the near C gates ) so they distance from landing to the general terminal area was short but the amount of time it took the actually work the planes in and out to avoid collision took forever.

Again whoever controlled the board that day just wasn't the right person for the job because it was very poorly executed.

Like the OP, I also had a similar albeit shorter experience out of LHR over the weekend. No delays on the field itself (no weather, no clouds, etc.) but it took about 60 minutes from taxi to takeoff given the number of flights waiting to takeoff. Interestingly we still landed 30 minutes early (even on a Westbound flight) so clearly UA and others have built this wait time into the scheduling (same as they do for flights into/out of JFK/LGA/EWR)