My max fps was unlimited and I had no FPS issues, but today I limited it to 30 not to waste power. But when I did this my framerate instantly dropped to 20. It wasnt going up nor down. I tried the 40 FPS limit, it gave me 25-30 FPS. Then I pulled up the limit to 60 (my monitor's max). This option gave me 40-60 FPS. After this I scrolled it to unlimited and it gave me continuous 60 FPS as before.

I think the 'Max Framerate' option just doesn't limit the framerate. It also reduces the FPS.

Note: I tested these in the same place and with the same video settings.

Sorry if I used wrong grammar/word.