I think the problem is that given the number of entry airports into the US, plus factors such as number of international arrivals at any one time, number of Border Control staff etc, our individual experiences do not really help .
All I know is that it always took longer to clear immigration in the US than the UK.
Especially frustrating when I had to fly to New York for a three hour meeting, and I had to queue at Border Control for almost longer than the length of my meeting !