I play PSO on my TV set, which I usually have set to 125% scaling in Windows 10. When I first started up NGS in virtual fullscreen mode on my 1080p TV I noticed how all the text was blurry and the performance was a bit rough even with all the graphical settings turned down. When I started it up in windowed mode with the resolution set to 1920x1080, I was getting this on my TV which tipped me off to something not being quite right.
When I tested it on my regular monitors everything was fine, which led me to the scaling options in Windows. When I set my TV back to 100% in the Windows display settings, the text was crisp and clear, and my framerate was a rock solid 60fps.
I'm assuming that with the monitor scaling set to 125% the game was rendering at 2400x1350 in both windowed and virtual fullscreen modes, and since this isn't an integer multiple of 1080p the UI elements weren't able to scale cleanly leading to the blurriness. The higher rendering resolution would also explain the performance issues that disappeared when the scaling was set back to 100%.