Did you try to jump from the battery starter wire to the starter selenoid (with ignition switch off or trans in neutral) just to see if the starter turned over faster? That would be the ignition switch. /forums/images/graemlins/tongue.gif
Had a similiar problem with my 71 Mustang in which starter selenoid would click when I turned ignition switch to start and the starter itself would just groan. Ended up being the switch. If that doesn't work and all else is right with tractor, a slow starter probably a internal problem with selenoid contacts or brushes/ commutator in starter. You might want to take apart selenoid to see if the contacts are burnt.
camman12