I recommend a min of 60 fps for rF2. Use "ctrl f" to view your fps ingame.
But if you really want to: in the player.json file you'll find this:
"Max Framerate":0, ==> change the 0 to 30
"Max Framerate#":"0 to disable (note: positive numbers only, we always use the 'alternate' method now)",
BTW: What works in one game doesn't mean it works in every other game.