Typing in Gmail had slowed to a crawl. After trying many things (emptying cache, restarting Firefox, signing out and back in to Gmail, etc.), I tried disabling Gmail Show Time. My problem instantly went away. I'm not saying that this proves a problem in the Add-on, but it sure deserves some attention. And if you find that Gmail suddenly slows down, disabling this Add-on is the first thing I'd advise doing.