It's almost certainly out of adjustment. Without an owner's manual, follow the linkage from the pedal. Eventually you'll come upon a place to lengthen or shorten the linkage, which is done by turning clockwise, or counter-clockwise a nut with likely, a lock nut, on a threaded rod. Turn in a direction that creates more resistance. You want to do this sooner, rather than later. It's likely the clutch is slipping, and you can't feel it. Slipping clutches wear out quickly.
However, based on your description "it slows down" - what should slow is the movement of the tractor - not the engine. Engine speed should remain constant if the clutch is slipping