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 fru........
I installed 5.5 with a 300GB RAID 1 partition (boot is also on this partition). It booted up fine the first few times until after I used a Live CD and accessed the array, and it became named /dev/md127 for some reason.
Now whenI boot into CentOS I get a kernel panic and different errors, once I got "invalid superblock", even though the array is fine (it didn't happen again, probably because I was sure to dismount and stop the mdadm array properly).