I nearly missed a connecting flight in Heathrow earlier this year, because it took them thirty minutes to get seven people through security. I kid you not - I was seventh in line at security[0], and they were only allowing one person at a time to unload the contents of their bag onto the conveyor belt. They had five or six agents all "working" there, and a peak throughput of one passenger every three minutes.
[0] International flights connecting in Heathrow have to go through security again, even though they've already gone through security at their original point of departure.