In my case things slow down when going Borderless. But performance is ok when going full-screen. My guess is that it doesn't like it when other applications compete for video memory. If that's the case, Square need to check their texture caching code.

As for small number of posts.
Have you consider that many people won't post their issues? Most players do not even know about this forum. Finally. consider that every unanswered performance issue is a lost subscription. This isn't an OS where people are stuck and there are a lot of competing games.

For every 1 person posting their technical problems, there will be a 100 persons not posting them.