I agree with you Chandler. The glow plugs should have no effect on the starter. If you cannot consistently get the engine to crank at a good speed, then I would be looking for something else. I assume you have tried jumping it to make sure the problem is not the battery? Maybe next you should get the starter bench tested to see if there is a problem with the starter or solenoid. Starters are usually not too hard to take off. Make sure you disconnect the battery before messing with it though or you you may be in for a shocking experience.
Before testing the starter, you might try jumping it directly by bridging the contacts with a screwdriver (one with an insulated handle - or maybe you can devise another safer method). If that consistently spins the engine then the problem is probably somewhere between the ignition key and the starter solenoid.