Let me first throw out the things people are looking for when clicking on a thread like this:

• Turning off the 'Psycho visual tuning' option in OBS;
• Make sure you remove any unused scenes and turn off the recording preview;
• Disable Windows 10 Hardware GPU Scheduler;

For me, whenever I open OBS, no matter the game i am trying to record, it always tanks it's performance. I don't have to be recording or streaming; simply having the application open would either consistently or sporadically tank my fps in any game. For example; running COD 1 (released in 2003 if i'm correct), my machine can steadily stay above 1k fps. But when OBS is simply open, it struggles to consistently stay at ~120fps or any fps i would cap the game at. Now there are a lot of different ways we can optimize OBS in order to run it somewhat tailored to your machine, but none of any of the options/configurations it supplies had as much as a difference in practice as the things i mentioned.

For anyone that is interested, here as some threads/forums talking about the issues;

https://obsproject.com/forum/threads/huge-fps-drop-in-game-by-simply-having-obs-open.142079/

https://obsproject.com/wiki/How-to-disable-Windows-10-Hardware-GPU-Scheduler

https://obsproject.com/forum/threads/game-tanks-in-frame-rate-even-i-way-surpass-the-requirements-when-i-try-to-stream-it.148366/

Hope it helps anyone dealing with the same problems! Please correct me on any misinformation if there is any. I am by no means an expert on any of this.

You could open obs as an admin so it uses more process on it