I was through Gatwick only recently but travelling back to Dublin and not to the US. The queues were reasonable enough considering it was summer season & it was only 3 days after the scare in Scotland.
The biggest problems were people who weren't prepared so didn't have all their liquids below the recommended size and in a plastic bag. Surprise, surprise! I've invested in a box of ziplok bags and I love these because nothing can fall out.
Don't forget to wear shoes that are easily kicked off and put back on again, don't lock your handluggage & smile. I did get frisked myself because the blasted metal detectors were set to sensitive and my underwire on my bra set the alarm off. Something to bear in mind......
All in all, it took me 20 mins to get through the queue, get frisked and get my shoes back and and walk into duty-free.
HTH