I have a gravely pro walk behind. 50" cut with a Kawasaki fc420v motor and is also a hydro. I have replaced all the belts and replaced hydro filter and oil.

Problem is, it moves very slow. Like a slow walk in forward and in reverse. I am at a point that I don't know what to do?? Is there any type of screen with in the hydro that could be clogged or anything I can check or may have missed?? Any help would be greatly appreciated. Also, sticker id on mower is, model-988068. Serial-000187