With vari speed mowers slow running is usually due to a stuck variable pulley or worn out belt.
Try pulling the brake pedal up.
If that makes it go faster then the clutch needs attention.
Do you have the owners manual for this mower ?
Some of them have fixed speed ratios which limit the travel of the GO pedal
Because these transmission work on balancing spring pressures, a misplaced tension spring on either the upper or lower belt will also cause it to run slow.
This will depend upon weather you have an electric PTO ( pull a button ) or manual PTO swing a lever.Also, I found the belt that runs the mower deck comes off of the front pulley. Can this be tightened or does that belt need to be replaced as well?