So I went to try to see the usage for a slave on builds.apache.org, and the page had no builds on it. I eventually noticed the "Calculation in progress" bit and thought "Oh, ok, I'll leave this up and check again later". That was a mistake. Now there are 30+ threads on the master like the ones in https://gist.github.com/abayer/88e390e3f0859f8b64e2 - i.e., a whole ton of HTTP POST requests to /computer/foo/timeline/data, all but one blocking on the one that's running, and the one that's running takes a long time to finish.
This means (a) that the build history page for a slave is useless and (b) that we're churning CPU/IO and, I'm guessing, doing so repeatedly without caching, since when I check it now, even an hour and a half later, there's no data on the page.