The users pc updates as often as possible. When he's using a slow pc it may be possible that his pc can't keep up and the gap between two frames will be made larger, because it takes more time to process a frame, meaning the sprite is moving more irregular. (This is the same effect when playing for example in an online FPS, where you have a high ping rate and you see yourself suddenly jump forwards).
However, users with a more powerful pc who can keep up, may not experience that slowdown because the gap between to frames remains small.

I would recommend leaving the UpdateSpeed calculation alone and only change the speed of your sprites to a value you're comfortable with.