If you shoot in AF-C and custom setting a1: AF-C Priority Selection is set to Release then focusing cannot slow down the frame rate. The shutter will fire even if the camera is completely out of focus.
Check your shutter speeds – you must be above 1/250s.
High ISO noise reduction need to be OFF. (shooting menu)
Don’t use auto ISO
Make sure that Active D-Lighting is not enabled. (shooting menu)