Somewhat a repeat of the above.

Starter runs slow because:

It is not getting enough current due to resistance in the cables/grounds (contact areas not clean or corroded cables).

Starter Brushes are shot, main contacts inside of starter solenoid burned and not allowing enough curent to pass into the Motor.

Some issue with the Engine that is causing it not to rotate easily.

I have read on this form that the Starters for the Turbos crank the Engine Faster.

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84 300D, 82 Volvo 244Gl Diesel