There is a relationship between hard starting/cranking time and cylinder ring wear. That relationship last with Dexta's until they wear until you get blowby and run-away.
A rebuilt engine will fire readily at starting RPMs even cold.
I have a rebuild Dexta that all you have to do is bump the starter and its running. I have several others that you grind on them like making sausage, heat them for a minute to get them to run.
Another trick I have learn is the original 1.7kw starter can be replaced with the 2.6KW key start stater and that tired engine - bump she starts- I found a Perkins gear reduction starter that will even beat that.
After they are running, there is little difference in power (for now) between the fresh and more tired engines.
Think I have addressed the answer and the work-around to hard starting, without even discussing glow plugs.