This is still a bug for me, even after almost three years and a few different computers. It's always persistant no matter hardware, drivers, os, etc.
What exactly happens to me is I lose 35-50 FPS from my solid 70 FPS and it never comes back. It happens *instantly* and for reasons I can not figure out.
The only way to fix it is to log to the character select.
I can look in a direction and the FPS will go down to 25, I log out right there on that exact spot I have terrible FPS, log back in, look at the exact same direct and everything's back to normal solid FPS with no issues.
What causes this? It can happen within 5 minutes of play or an hour but it always happens and I have to constaly be logging to Character Select and back in to the game.
So I tricked the system and turned off Vsync. So now instead of dropping from 60 to 20 through 25 I now instead *when I'm FPS bugged* go from 100*** to 70-75. So it cures the symptom but it doesn't fix the problem. It is still there.
*** It seems 100 is the Max that the in-game FPS Display will show becuase it is a solid 100 never goes to 101 or higher and never goes to 99 or less. I'm sure it's far higher, most likly ~150 FPS [with my rig specs] when it's truly working and the Bugged FPS (like mentioned above) is ~75.
So shut off Vsync and let the video card go wild pumping out frames and when you get bugged you'll hardly notice. lol