If belt does not fix it then I would think the drive is going out. Kevlar is for breaking very few belts break they wear. Unless you can get one very close to the same price I think they are too pricey.
On the backside of the trans is a lever you pull out so you can push the mower when it is not running. Make sure nothing has wedged up in there and pulling that lever partly out, it will make the trans pull slow.