Chrome with Windows 7, no other bugs, no other lag.
I have the golden cookie sound on, working in another Chrome window and switching to the game when it sounds. That worked fine for a while. It still works perfect for the cookies, but it lags for the reindeer. I hear the sound when the deer is almost at the right edge of the screen, so I can't catch it.
The lag affects about half of the deer. I think it started to happen after I received the Eldeer achievement, but that may be a coincidence.