While something else may be going on I would think the extreme cold haas most to do with it.
Metals contrac and depending on the metal some will contract a great deal.
This decreases the amount of clearance between metal components, resulting in greater friction forces brought to bear when you try to move them. This happens not only to the engine but the starter and all other moveing parts in the vechile/machine.
Add to this mix the fact of how diesel is combusted to begine with and the fact that cold reduces the amount of power a baattery can produce and you get a really hard /long start condition.
Sorry for not giveing a more detailed explanation But I ain't feeling so chipper this morning, the cold has me not wanting to start too good either.