John Deere 955 Is Slow

Hi all! New to owning a tractor and I bought a nice John Deere 955 with around 480 hours, 1992. The problem I am having is that it is very slow, around 3-5 mph and the manual says top speed should be around 11. All fluids are good, no leaks that I can see. I have it in high gear, placing it in low gear slows it to a snails pace. Maybe linkage problem? I don't know and looking for ideas before I have to pay big $ to a dealer. Thanks All

John Deere 955 Is Slow