My FPS on Minecraft stays at 120 even though I have unlimited FPS

This could be completely normal as the fps depends on you're device's refresh rate even if you set max framerate to unlimited, unless you're device used to run higher fps on Minecraft.

My computer used to do 300 fps now it has shrunk to 120 after changing a 3D setting envy control panel

@unknown Yeah sometimes changing performance settings/3D setting envy can change you're fps, if that's the case have you tried resetting you're settings including the 3D setting? And you should also make sure to keep you're graphics card/graphics driver updated to avoid any changes in performance.

Disable VSync in Video Settings.

Please attach a screenshot of this occurring while the F3 debug screen is enabled.

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thanks for your support the problem are solved