hoo boy.... +- one or two slows? It's all relative, described that way.
Try looking in the chassis cavity under the tilting seat and getting the full mgfr's model # off the ID decal/plate post back here with that info. mebbe we can look up specs and other details to find out how fast it should go and if there are details available for machine repair/adjustment.

Hard to read as there are white triangles all across the label. Here are the numbers I could read. Model# 14AY806, below that there was 890889. Serial# looks like 7B00135. Manufactured by MTD.

No joy. MTD could not recognize those numbers.

if the hydro tranny has been slowing down over time, try an oil and filter (if so equipped) changeout. Somewhere on the tranny there will be an ID tag giving the mfgr's name, model number, etc. Use that info to find out what is involved in changing the fluid. May also be some info in your mower's operator's manual.

If it's fast enuff in reverse, but slow in forward, take a close look under at the linkage - may be some bushing slop/wear limiting stroke movements..... or a balancing adj in the case of a "teeter-totter" type control... good luck.

Hydro Gear Model# 618-0249.

On the tranny - call Hydro Gear Consumer/Dealer Support: 877.728.7410 - give them that model # you found and ask them for specs on the fluid and how to change it out.