Hi there,
Your starter is a 4-pole motor. it's possible that one brush isn't doing it's job and is causing the starter motor to only have half the power... this could be causing your slow turnover. The only problem is you will have to remove the starter to check it. And, if you have to remove it, you might as well replace the brushes in it, or install a rebuilt unit.
Also, you might try removing all the glow plugs and turn it over on the starter to see how fast it turns. If it still turns slow, it's most likely the starter. While the glow plugs are out, do a compression check. You should get at least 300lbs of compression on all cylinders. If you do, remove one injector at a time and let it spray into a can. See if it REALLY sprays any fuel.. If it does it should start. Use a hair dryer, blow hot air into the intake. If it has fuel it should start with the hot air in the intake manifold, even with no glow plugs operational - this is a trick I have used to start an old tractor. The particular tractor I use this on, an old Ford diesel tractor, only has two glow plugs, both in the intake manifold to preheat the air, and it doesn't like to start on cold days unless it gets a little help from the hair dryer.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C