I've had this issue since Ubuntu 9 and now even on Linux Mint 17 no matter what latest version of the Nvidia proprietary driver that I use. It just becomes slower and slower and only gets better after rebooting or restarting Xorg.
Someone here has it as well
https://askubuntu.com/questions/822449/graphics-becomes-slow-after-a-while/917886
It's very fru........