I had left my browser open at the beta review screen (where it says "there are no more items left for you to review") while I was doing something else. However I soon noticed that the PC's fan was struggling, and sure enough checking Task Manager showed that the browser was consuming CPU. Navigating to another page (e.g. Questions) made the problem go away.
Is there some reason why there is code on the page that is continually parsing the same string? I couldn't see anything obviously happening - I wasn't even interacting with the page and it was still consuming CPU.