Technically it is recording for the preview in OBS, which I think goes on even when OBS is not visible, and I assume involves constant copying and downscaling with whatever FPS, but it shouldn’t be encoding until you hit record. but if you check the CPU usage of OBS, maybe also GPU with nvtop, you can see how much it affects your hardware.

I would imagine it’s not too bad, but still it would be better if it would let the machine idle as it should.