Here you go.
https://obsproject.com/forum/resources/how-to-make-high-quality-local-recordings.16/
If you don't read and follow the guide, we can't help you.
That guide is linked often as it provides the lowest system-impact while also providing the best quality video that can be captured.
As noted in the guide, you can set the encoder preset to Ultrafast (making CPU usage minimal) and unlock the bitrate to compensate for the worse-quality encoding. Minimizes system impact while recording, but creates much larger video files which can later be re-encoded with a more efficient preset to get the size down if you intend to keep the videos for archival purposes.
Doesn't work for streaming, but as you're just recording, it should work great. Now read it, and follow the directions.