I've never waited 120 mins at LHR. 90 mins once. It's generally around 30-60 mins at LHR for me when arriving in the morning (e.g. at the peak time of arrivals from the US).

Fast track is a strange thing. The queue certainly has fewer people, but there are generally fewer UKBA agents, too. When in one queue, I'll notice someone who joins the other queue at the same time as me, and more often than not, we reach the front at about the same time.