When the F3 screen is open, you have to move the mouse a lot faster/farther in order to look around. It also keeps drifting after you stop moving the mouse as if there's a delay between mouse movement and screen update.
Comments 2
You're probably using Windows 10. See MC-83488.
Sure not to have pressed F8 or bound the "Cinematic Camera" to F3 ? Check the settings under Menu -> Options -> Controls.