When we have a lot of folks trying out our machines at shows the one thing I noticed is that people try to treadle too fast at first. It is a rhythm thing not a race to the finish.
The one thing that I was able to say that seemed to help folks treadle a bit smoother is to think of it as more like dancing then like running a machine. Work with your partner, you don't just decide on a speed and poof you treadle that fast. With that said, some of my machines naturally run smooth as silk slowly, while other hum along at a good clip as their natural rhythm.