Record smoothly on Mac?

I would love to record some gameplays. I also have an average of 80-100fps in games like Portal or Minecraft. But as soon as I start recording, the fps drop to 5-20 fps. I tried QuickTime and OBS but nothing worked! Do you have any tips?

Just a tip. A Mac is not designed for gaming.

I already know. But I have nothing else but my old Mac. So that's not really helpful.