Some builds taking several times longer than normal

I've noticed over the past week or so that all the builds (PPA and Ubuntu) will occasionally freeze up, often stopping for well over half an hour. For example, I uploaded 3 mplayer sources, the only differences being the changelog. They built in sequence, but one took 5 times as long as the others. That one appeared to hang whilst removing the build-depends at the end, but eventually woke up. Timestamps in another build log show that it took no more than a couple of minutes, yet Launchpad showed it as being in progress for over 45 minutes.

You can also see, looking through https://edge.launchpad.net/ubuntu/hardy/+builds?build_text=&build_state=built, a number of sets of 15 simultaneous builds (ie. one for each buildd) that take over 45 minutes, while everything around them takes less than 10.