I doubt you would notice much a performance hit if any. What you’re losing out on is reliability. You playing games and downloading/executing stuff causes extra processes to run that normally wouldn’t on a server. The more processes running the more chances for a glitch in the matrix to crash the machine. You could also do an update that breaks compatibility with other programs (not so much an issue with Docker). The biggest point of a dedicated sever at home is the fact that you set it then don’t fuck with it - This leads to higher reliability.

Any computer can be a server and in most cases we ain’t pushing em very hard as we learn anyway

I cringe at the thought of putting pi hole on a gaming PC, you must live by yourself and have nobody else that relies on the internet. You shutting off your main gaming PC would shut down the internet for the whole household.