If I get 20x11x9 will I lose speed I have 20x11x8 on my Banshee.

 

so long as the relative heights of the two tires are the same, the 1"width would make little, if any difference in speed. The catch to this theory is that the actual height of a given 20" tire can vary with the manufacturer & also the tire pressure one runs. Some 20" tires are actually slightly taller than claimed, many are shorter. Unfortunately, the only way to really know is to mount the tire, inflate to your desired psi, & measure. But, regardless of all this, the width will have negligible effect on speed
Long answer to a short question.