It doesn't happen all the time - seems to be intermittent, but when it's happening, it'll go on for several minutes at a time. When it does happen, the activity monitor shows the browser process using basically the entire CPU.

Sometimes but not always, reloading the page fixes the problem; it does seem to be more likely to happen when the page has been open for an extended period, but sometimes the problem persists after a fresh page load. (There's one project that I use to organize most of my work, and I'm liable to leave that tab open to that project without reloading or even changing the sort/filter for weeks on end.)