This video shows 4 hoppers in total the first 2 hoppers on the left are pointed downwards and the second 2 on the right are point sideways. Each hopper is holding exactly 64 items and each of the hoppers are un powered at the exact same time and yet some how the hoppers which are pointing down on the left are a lot quicker then the sideways hoppers on the right. you can see in the video proof of this. Hoppers no matter what direction they are pointing should have the exact same speed.