Currently F11 will toggle on/off UI but if someone use Fly Wing / Teleport, some of the UI would be visible again. I'm not sure if this would be possible, but it would be good if F11 would toggle off the UI indefinitely till the person press F11.

Major reasons:

Ragnarok Online Client is REALLY resources wasteful, cause every single UI is updating in real-time, which lowers the client FPS for nothing.
Disabling UI means every single resources can be focused on the game FPS (performance)
Secret Information: Someone with High Refresh Rate monitor will have higher FPS. This is cause the game seemed to always try to match in game FPS with the Monitor Refresh Rate
This ensures that someone with high refresh rate monitor will have an easier time to get their real FPS (120, 144, 240, 360, 500) cause of disabling the UI
Ragnarok's gameplay is mainly walking around and killing monsters. While the 2d sprites (character, monsters) will never really get to high FPS, the 3d environment will hugely benefit from higher FPS. This makes farming with high FPS feels way smoother and responsive.

Proof how disabling UI increase my 40 fps to around 120 (my monitor refresh rate):
https://i.gyazo.com/474ad478fb84e493c2d273e8401e050c.mp4