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.