Definitely not as big as it used to be. Lineage, Graphene, and Pixel Experience are just about the only players left. Graphene doesn't officially support anything other than pixels actively supported by Google though.
No, it's not at least in my case. Actually GSI Lineage is a little bit faster due to less bloat than stock. But performance could be different for different devices.
Note: I experienced some bugs but all of them were easy to fix.
I have two Xiaomi devices and use custom roms for both of them. While there are pages for them in the XDA forums, most of the new updates and developments of custom roms and kernels, at least for Xiaomi devices, happens within telegram groups.
You could just try searching for ROMs and going to their git page or website and get a guide on how to install specific ROM. I recommend Lineage OS since it's the one I used for two~three years and because it doesn't come with Google shenanigans
Which mobile do you plan to use? ROM support has declined dramatically after ASUS and OnePlus shenanigans. It's Nothing, Google and Xiaomi/Realme/Samsung in Europe who have any support