We use Cruise Control on our Continuous Integration server and we have several ways of getting alerts about the status of the build, including email, RSS and the Cruise Control web interface.
Recently we noticed that the web interface and the rss feeds were taking very long to respond, on the order of 1 minute or more. After poking around, we realized that we had hundreds of serialized builds still on disk.
$ cd ~/OurCruiseDirectory/projects/OurProject
$ rm -r build-*
Then, to make sure this doesn't happen again, we edited our OurCruiseDirectory/site_config.rb site_config.rb to decrease the number of builds we keep:
BuildReaper.number_of_builds_to_keep = 20
So it appears that the time Cruise Control responses take is directly proportional to the number of builds saved on the server.