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.