The nut is not responsible for your loss of 3mph....many things could cause that - but not the nut, unless it has come loose.
Have you been back in to check things after you noticed the loss of top speed?
Stig
To my experience roller condition definitely contributes to a power loss.
My Agility started loosing power uphill, and I recently opened the variator to check the rollers. I found some dust inside, and two rollers had light flat spots.
Not having new rollers handy, I took a chance for the time being, and applied a thin layer of CV joint grease on the rollers and just put it back in place. Since I did this the power loss uphill is completely gone, and the scooter reach the top of the hill at higher engine RPM’s.
I guess the grease at the inside of the variator will gather dust, and could cause other problems. But for the time being I think I’ve learned that a combination of dirt at the inside of a variator, and slightly worn rollers can cause a loss of power in general.