has it got a battery isolator switch on the overhead guard?? a bad one of them causes slow cranking, also the wires to it are super long and cause a voltage drop

if the battery checks out ok and the cables are good then you may have burned up another starter. I would take it off and have it checked out.
that's why i said to check the glow plugs and the circuit that controls them. If the glow plugs are not working correctly it will cause you to have to do long extended crank times just to get the engine to start, especially when cold starting. This can burn up a starter.
The glow plugs are there to heat up the combustion chamber so it will start better, quicker, if they are not working correctly the engine will not want to start up forcing you go continuously wind the starter till it does start. The starter will get too hot and cause the windings to fail. ie: you get a slow crank as you put it.

thanks for the input,
By slow crank i mean its acting like a dead battery will crank super slow and hard. didnt check glow plugs will look into that, checked cables, removed serp belt still same super slow cranking

replaced starter and even battery and still slow crank?
check the cables, sounds like they may have some high resistance in them.
another thing to check might be the glow plugs, they should ohm out at 0.2ohms.
also you can test them by removing them, then connect to the harness and power on and check, they should glow red within 10secs. (make sure you have battery volts to the glow plug harness when key is on.
bad glow plugs will make it very hard to start.