I have been to LHR, but cleared customs at a different area as I was transferring on to EDN.
As for ORD, in their international arrivals building, I would say they have around 30- 40 or so. My transatlantic flight came in at an early time (AA flight 99, or 98 forgot which one, from LHR) right before the morning international arrivals came in. With a fully loaded 763, we got through immigration in probably about 10 minutes or so.
For DFW, there is probably around 10-15, it is horribly understaffed especially at peak times for American.
YVR has a good 30-40.
I don't remember for LGW and ZRH, it's been so long
Give me a break, I created this username when I was a kid...