When you say "noticeably slower," do you mean it's just less power than originally, or does it barely move under its own power? It's a little unclear as to what you mean.
If it's just an apparent performance degradation, I'd tend to blame the motor first; mainly due to overheating if the vehicle has been run in grass constantly, or more than one back to back.
If it barely moves under its own power, but sounds like something is spinning like it should, start by checking wheel pins, hexes, driveshafts, spur/slipper settings and tension (sounds like you did the slipper already but it can't hurt to make it good and snug, then back it back off to where if it was if there's no change), etc. Bad gears usually make horrible grinding noises so you're probably OK there.
Good luck! Post up with any other clarifications, questions or updates of success or failure!