Capture cards (for making clips), help me understand these

I love making clips and I THINK I need to look into capture cards. Maybe you can tell me if i do and which one to get.

-On Xbox, the DVR clips look terrible and I think it creates lag so I would like a better way of making Xbox clips. Not super-concerned about this though as I’m playing more on PC now.
-On PC I like playing in 4K/60FPS and if I run the DVR I think I’m going to start running into performance problems. I’d like a card that can handle this independently, with no load on my gaming GPU.

I really don’t know what I need so any advice here would be a help. Thanks!