[denisond3]

You dont say what year your VW is.
I would start by checking the ignition points. If they are out of adjustment, it might be trying to fire too early, and the combustion pressure is making the engine turn slowly. A good way to diagnose if this kind of thing is happening, is to unplug the wire from the coil to the distributor, and see if it still cranks over slowly.
If it still cranks slowly, I would suspect there is too much greasy crud on the commutator of the starter. A new starter might fix the problem, but first I would remove the starter and dismantle it - to look for the dirty commutator. Also look for a well worn bearing that is allowing the rotor to rub against the stator coils. There is a tremendous magnetic field when the starter is working, and it may be pulling the rotor against the stator coils.
Finally, have a look at the wiring from the battery to the starter. This heavy cable may look fine, but there can be corrosion under the insulation that is invisible. The only way I know to really learn the condition of the wire, is to cut the insulation back - and installing a new cable is then needed.

And the negative cable wiring is just as important as the positive cable wiring. I only dimly recall, but I think the negative connection to the engine from the battery goes through a bare braided cable under the car. This braided cable might be about dissolved from the years of water and salt.
Good Luck with your beetle.