I just started playing this game, and it seems good (though the writing and voice acting are...rough at times) but every time I get an achievement the frame rate takes a huge hit and once I even experienced a full system lock. Other notifications (such as friends logging on to Xbox Live) do not cause this issue. It's gotten to the point where when I see the frame rate drop I pause the game, let the achievement pop and recede, wait like 10 seconds, and start playing again. It's not a huge practical problem, since there aren't that many achievements and I can just pause and wait it out, but I've never experienced anything like it and it seems like something that should have been patched if others experienced it.
Am I alone in having this issue? The frame rate has been fine for me otherwise, and like I said, the game seems good.