I was also experiencing a very sluggish desktop (using KDE and some GTK apps). gtkperf reported some very slow 12.5s for 100 test rounds of everything.
Originally Posted by gradinaruvasile
Does your GPU have dynamic clock setings (multiple performance levels)? Check in nvidia-settings -> PowerMizer.
Taking a look there I noticed that nvidia-settings reported GPUPowerSource = 1 (Battery) even though I was on AC. The PowerMizer mode was set to "Adaptive". So I unplugged that cable, plugged it in again. nvidia-settings still showed to be on battery. I restarted nvidia-settings, and then 2d performance was suddenly great again! The tool still reports to be on battery, though.
I'm not sure what it was that did the trick (un- and re-plugging in the cable or restarting nvidia-settings, or both), but I'm happy that the system is usable again. gtkperf reports some reasonable 2.5s now.
EDIT: I'm not using the 256 driver yet, but 195.36.