Q: What's the performance impact on this, especially on windows?
Basically negligible. Secure DNS might be a bit slower and you might feel some impact on low end devices.
Q: Also what would happen if I installed a Windows gateway, using routing and remote access services, and then installed portmaster on that?
You'd probably be cut off as incoming connections are blocked by default. Please place a config with exceptions before install or have (virtual) physical access when installing.
Q: Oh and can I use this in conjunction with wire guard? How does it play with other vpns.
We have a whole page on that in the docs: https://docs.safing.io/portmaster/install/status/vpn-compati...