Recently I too have been having the lack of power on my tractor (19.5HP). Full speed was anything but and reverse was really slow. The engine runs full speed and is in good shape. I sprayed some belt dressing on the drive belt and the power is back. It is kind of tough to get at - I lowered the lawn mower deck and got the tractor up on ramps and I could see the drive belt way up there. I let it run with the brake on and this allowed the belt to continue turning while I sprayed - the little straw on the spray can helped direct the spray to the belt. This stuff makes the belt stickier. So it seemed that as the transaxle is loaded up and needs more power, the belt would start to slip and limit the power transfer to the transaxle. Probably time to replace the belt (perhaps it has stretched at bit) but I thought I’d share my solution (at least as long as the belt dressing lasts) with others.