Hustler Fastrack loss of ground speed

My mower slowed down all of a sudden. Still cuts good, but top speed is down significantly. I have changed the belt that drives the hydraulics and i have changed the hydraulic fluid and filter. No improvement. It does seem to creep backward ever so slightly when i put it in neutral position just before setting to park. About the only thing i know to do is adjust the linkage but am hesitant to do so. My dealer has looked at it. He changed the belt and repaired small oil leaks that he thought might be causing the belt to slip. No improvement. I changed the fluid and filter myself today (9/14/19) with no improvement. Anyone have any ideas? It tracks pretty straight and i don't notice any change other than the ground speed is slower. I do not know of any changes that were made that could have made it slow down.

Have you performed an oil & filter change & tune up recently, replaced air & fuel filters, spark plugs, cleaned underside of deck, sharpened blades, inspected belts, greased mower, checked tire air pressure, etc.?