I had PI3 Running perfectly smooth for a couple of weeks ago, when i decided to update my videodrivers for my ATi Radeon x1950 Pro.
Then after the update, the program shows me the message "OpenGl Hardware Acceleration Is not available, Sofware rendering must be used"
PI3 now runs like dirt in the gears and It's impossible to watch any big effects with any resolutions over 100*100 or it will lag like hell.
Oh, just thought of something else. In pI3 Prefs, OpenGL page, check the "do not request accumulation buffer" option, click OK to close prefs, then close pI3 and launch it again.
If that doesn't work... If it were my system, I'd probably go to the previous version of the drivers (next-to-latest) and see if that works, then work my way back one version at a time. Worst-case you end up with the same version you had previously.