I don’t think this is how it works? If the servers are stressed then Fortnite here would be having the same issues. Now unless Forza and Fortnite are both tied to different servers then that could be a thing, but then that’s not so much a Fortnite issue. GeforceNow can run into things like this where certain regions have more issues at times. I dunno how Microsoft does their stuff though, if you can even change what servers you are connecting to.
Forza you would get some more artifact and pixelated stuff though, just because of the speed the game is going at. So you’re going to notice the stream trying to update each pixel as fast as it can.