When ever I record using Quicktime player (version 10.0) it records at a much lesser than the 90 glorious frames per second I get when playing Minecraft. Even when I'm recording my fps>50 but quicktime only records, I would say 8fps. I tried using Screenium, that was even worse, even when recording minecraft at 640x360 resolutuion. Everybody else says those work perfectly, but they don't. I have 4G of RAM, and dedicate 364mb to Minecraft, so it's no that I don't have enough RAM, unless 3.6G is not enough.
Maybe try changing around the recording settings? Sometimes you need to change the quality when recording because it can really affect the FPS in the recording. Let me know if it works, if not I'll run some tests on my old iMac
There are no options to change the quality or fps. I have tried using my friend's Screenium, it works better, but I tell it to record at 60 fps, but it records at 23. I generally run minecraft at around ~100fps.
If you hit new screen recording and look to the right of the screen it brings up, there is a little white arrow. Click on the arrow and you will see an area where it says quality and you can change it there. Also, are you running other things while recording and playing Minecraft?