The curtain speed is a fixed quantity, it is the same for all shutter speeds. Unless you have instruments to measure curtain travel time and shutter speed (effectively, slit width) you can't really tell what's going on, except by making test exposure - a crude method at best. If you can actually see that the curtain is sluggish, it's way out of whack. What may effect it at slow speeds (large slit width) would be poor lubrication in the drum and roller (under the shutter speed dial) and associated timing mechanisms.
Originally Posted by Lamar
These shutters are pretty simple once you understand the theory and get used to working with smallish mechanisms. Keep practicing, you'll be fabricating your own curtains in no time!