Question / Help Recording stops starting the game!

[Maggintosh]

I just wanted to record a "Let's Play" for YouTube with OBS. Recording my desktop is no problem at all. Everything is working alright until i start the actual game. In the video fps drops almost to 0. Sometimes even the recording breaks completely when the game starts! What is the matter? Cannot imagine my hardware is too slow to record videos even in 720p. I tried out almost every possible configuration in OBS...nothing works! Please help :/

Here are my system specs:
Intel Core2Duo 7400 @ 3,4 GHz
NVIDIA GeForce GTX 560Ti
8 GB RAM DDR3
Windows 10 Pro