Have you checked the condition and tension of the belt yet ??
As it wears down it rides further into the pulleys and so it's driven slower by the clutch since the diameter in now smaller and it also drives the blades slower for the same reason.
One of mine seemed to be working poorly and I found out that the tensioner pulley pivot had seized. It had a plastic sleeve in there but the grease fitting and grease only reached one side of the sleeve. Rust build up on the inside stationary stud pushed the sleeve out into the rotating sleeve and bound it up.