When I was in game and recording, The fps in the top left of my screen would stay around 120. But when I would play the video back, it would look like it was about 20 FPS. It aggravated me very much because it seemed like a lot of people had this same problem but nobody knew how to fix it. Some people say your computer may be slow or your write speed on you HDD/SSD is slow. In my case I have a Core i7 processor clocked at 3.40 Ghz, an Nvidia 670 GTX graphics card, and a 128 GB SSD that I record the games on. The write speed is over 300 MB/s, finally after weeks or searching I found a perfect article on how to fix it.
https://forums.station.sony.com/ps2/index.php?threads/redcats-guide-to-recording-in-planetside-2-dxtory-ffsplit-obs.80232/
If you follow the steps in there I can almost guarantee that it will fix the problem. But I can not be 100% sure because everyone's computer is different.
I really hope this does fix it though, if it does, your welcome. If not then I am sorry.
Have fun guys :D
Checkmark Synchronous Surface Lock in your dxtory.