Three reasons for a heavy clutch on the 600
1, Cable routed incorrectly, putting a sharper than acceptable bend on it.
2.Poor spurious cable the OEM is best .3.The worm gear bearings in the Sprocket cover are shot4. Someone fitted EBC Heavy duty springs to the clutch.
Wear on the clutch could cause it but would be uncommon.1 and 2 would feel fine with the sprocket cover detached, but would not work well under load.
I would say check the cable route first as per the service manual.
Put in an OEM cable and grease the worm gear in the sprocket cover. There is a how to doc in the downloads section on greasing the worm gear bearings.
If still a problem, replace the springs in the clutch with OEM.