Steam

Despite it being in the FAQ, all it actually says is that they try to cause as little performance loss as they could. No idea how much that is, or if people will turn it off to get a few frames back.

It does say that without a GPU the performance is considerable.

pc has already had this with Xbox game bar and the perf impact isnt really noticable. if I recall correctly modern gpus have some dedicated chips for video rendering that are otherwise idle. I would assume there is some hit to vram that could hurt if you're limited on it, but for most people it should be fine.

They discuss performance in the FAQ section of this article.