The "Motion Blur" setting is a camera setting that is accessible via the F2 overlay in Experimental Rust. It makes the game seem more realistic by blurring the world around the player's viewpoint as he/she turns, just like in real life. Motion Blur has a big downside however, and that is in-game performance. Expect your game's FPS to dramatically drop if you don't have a pretty good graphics card. If you are having trouble with keeping your FPS up, it is best just to leave Motion Blur off. Motion Blur can be off or on by default, depending on the preset of your game before it starts. For the Fastest and Fast settings, it will probably be disabled by default. For the Beautiful preset, it will probably be enabled.