First double check you don't have a dead cylinder. Pull off one plug wire and start engine then do the other cylinder and see if engine runs the same on each cylinder. What you describe is low power from a dead cylinder.
If the clutch is slipping it will burn up fairly quickly.
Take the deck off and remove belt. If the belt is slipping it will do it on the clutch pulley. Look for burned spots in the belt. Check all spindles and idlers on the deck.