Hello, I've previously used Dxtory fine, and this has only started occurring recently. When I'm playing Left 4 Dead 2, while running Dxtory, the game runs around 30 fps, without recording. When I shut it off, or use Fraps, I return to my normal levels (60+). Note this is before any recording happens. I've fooled around with the settings in Dxtory and l4d2, and can't seem to get any differences to occur. I'll edit after testing with a few other games within a few hours.
EDIT: Turning on the Delay Hook option does seem to fix the issue. It looks like 2.0.127 might have something to do with this, as after rolling back to 2.0.126, I got it working, and once I updated back to 2.0.127, I can't.
It's possible you have turned on FPS Limit. It works even when you are not yet recording.
(http://i.imgur.com/Egbtchis.jpg) (http://i.imgur.com/Egbtchi.jpg)
I do have it on normally, but it was set to 60, and I've tried with it off anyway.
I've updated my original post with what appears to fix it, and some more information for those who care.