I love this software, the performance hit is almost unnoticable. However...
At viewing, and even after export, the footage has very short frame hangs. It seems like the video is going to race ahead of the audio and so the video is halted. This has happened in various games.
Eg. I played BF3 today and recorded everything. I can run the game in 1920x1080 with High settings with no problems; FPS stays at around 60 all the time. I also record at 60 FPS. During gameplay, there were no stuttering or anything, just smooth gameplay.
After I got to view the recorded footage, it frame-hanged for a very short time at some points, with around a second between each hang. Often in a heavily performance demanding scene.
What I noticed during viewing, was that the Bitrate keeps jumping up and down during a demanding scene, ranging from thousands to tens of thousands of difference between the past seconds. I think the frame hangs are somehow related to this. The bitrate at recording/viewing is insane; around two hundred thousand. (Press the "i" when viewing from bottom left, it shows the bitrate and other info.)
Lowering the graphics to the lowest possible makes the hangs go away, though not completely. They become even shorter and don't come up as often, but the difference is clear. You could say this is the fix for the problem, lower your settings, but you don't want the game to look bad.
Other games include DiRT 3, Mass Effect 3, Batman Arkham City and Total War Shogun 2. All these performed during gameplay smoothly with no stuttering or lag, but they all also suffered from the frame hangs during playback.
I hope this gets addressed, since this is by far the best recording software I've used during the past ten year of gaming.