If you're running FSR2 at 100% screen resolution it will drop your FPS because it's also doing some behind the scenes post-processing and AA that don't happen with it turned off (turned off uses a different AA method)
So you manually have to lower the render resolution in SF when you enable FSR? That's a really odd way of implementing it, but I guess that explains the lack of "quality" setting for FSR that you typically find in pretty much every other game.
Maybe it's just me but FSR behaves so weirdly in this game.
If I enable it when I'm indoors, it looks pretty good, even at like, 70%.
Out in the city though? Looks absolutely fucking awful, even at 100%, it's like just having it on makes the distant objects look bad, I don't get it.