That tells me that somehow the drivers were not loading at all or the wrong driver was loading first causing the device (GPU) not to work properly.

I'm not entirely sure how you would find out if an update wrecked things.
There have been a few times when I was running other distributions (other than what's in my profile) that updates created graphic problems for me and or other things.

If this performance problem occured after an update or an upgrade to Mint than it was most likely something in the update or upgrade that misconfigured or broke the way the drivers should have worked.
-::-It's happened to me.-::-