optimize FPS

When it was laggy, there will be a pop up said "optmize fps blablabla" So, How do i change it back to normal after i accept it? because of that, I cannot see players movement and the weapons theyre holding, only thing i can see is that they walking around like a block without swinging their arms and legs. I hope this can be fixed, or this is only my problem. But pls tell me how to fix that. THX