Generally go slow to learn, then faster as you gain skill and confidence. I think there are real advantages for using speed while stropping. The main being that a faster stroke from point A to point B on the strop will have a more even and balanced effect on both sides of the blade edge than a slow stroke, thus a better shaving edge.

OTOH, if you think you are getting good results with slower stropping, then who could argue with that? And then again, what you now think is a good shave could still be better yet. So it still could be beneficial to you to strive for more stropping speed as you get better.