Traktor suddenly slows down drastically

First thing that comes to mind: have you done any mapping adjustment with keyboard mapping or hardware mapping, maybe something you pressed accidentally has caused a direct change to tempo?
I’m assuming you mean the tempo has changed in the software and you’re not getting audio glitches? If it’s more the glitchy route & you’re in full screen mode, do a test without it in full screen if you can, see if that helps.