As many people experianced with playing Dungeon Defenders on high-end systems I had the same issue where there was a total lockup on the amount of FPS I was getting. There are -alot- of threads and stuff out there with fiddling in configs and settings but there was only one actually usefull one (to which I can now no longer find a link unfortunatly) of one guy stating it was his ASUS Gamers OSD that was causing all the issues for him.
I was desperate enough to believe him and figured: Don’t use the stupid program anyhow as it’s buggy as anything so may as well remove it. Needless to say it actually did work (otherwise there would be no post here). It is now also included in the FAQ on the techsupport forum on Trendyent.com