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
It's very frustrating that this issue has been recurring with all OS's, drivers and different Nvidia cards I've had with no resolution. I am almost considering going ATI if that driver doesn't have the slowdown issue