I can't really speak to the first part of your message, since I've never really tested anything like that, but at least anecdotally I've never really noticed any additional click latency in Linux compared to Windows.

For the second -- FFXIV works flawlessly _once you get it set up_, but there's a couple tweaks to deal with to get there -- the launcher in particular (the default/"modern" launcher doesn't play nice). Games with separate launchers in general seem to have more issues/be more finicky in Proton. I don't personally use ACT, so I don't know if there's any real issues with setting it up, but I know others on Linux who do so it's at least possible. For your reshade concerns, GShade[1] comes with a linux install script that I've had no issues with.

[1]: https://gposers.com/