You should check out pixelprospectors links and examples on how to make a trailer:http://www.pixelprospector.com/trailer-creation/

There are plenty of ways to make a trailer. A lot depending on the type of game. For 2D or 3D generally. you can render an animation and or screen record footage of your game. You can then edit the footage if needed, and add in some sound.

I'm also using OBS - while it's rather 'heavy' on CPU, but it's the best variant I have at present (I've triet lots of that struff but none is free enough or meets my screen recording requirements). My computer is not brand new (a few years old, I think, less than 6), but everything works smoothly. It also depends not only on computer configuration but also on HDD speed. Maybe, try RAM disk?