Slow turning over 6.5 GM Diesel

I had the same issue with a truck I had many years ago. Same thing you describe.

It was the starter. The field windings had a short in them somewhere causing a partial bypass of the current. This effectively reduced the magnetic strength thus causing a slower starter RPM.

Replace the starter and don't buy a cheap junk thing from China. Get a good reman from a reliable company such as Bosch. Or get an OEM - AC/Delco.

Replacing the glow plugs won't make anything turn over faster except your trusted mechanics bank book.

And ditto on the connections. When they look good take them apart and clean them anyways.
Don't forget the one where the battery cable connects to the starter! That's a suspect too. I had that happen once myself.

I agree to double and triple check all connections. Although starter motors can get weak over time, it would be so gradual that you wouldn't be able to say it started recently.

My recent hard start problem was dirty battery cable connection on the engine block. Removed cables and shined them up, put some dielectric grease on them and she fired right up. You might want to check them even if they look clean.

Don't take it back to "trusted mechanic". it is well known that diesel engines need to turn over a certain RPM in order to start. Had same issue with my Dodge RAM... it was starter that needed replaced. In my case the truck started just fine when cold but when hot it would not always start. Let it cool down and it started just fine.

Yes the starters start to drag when they get older. And yes the 6.5 is notorious for this because the high compression they have.

Don't buy cheap junk from Kragen or whoever or you will be sorry.