I think it takes significantly longer for an engine with the sheer mass of the Cummins, to come up to a totally uniform operating temperature. That's one reason I really hate to use my truck for routine around town driving.
My wife's '05 Ford Escape will come up to operating temperature within one mile of operation, at least according to the guage - but the Cummins takes a good 10 miles to do the same - and undoubtedly much longer for the deeper down areas to fully normalize.
Add to that the time it takes for related components, oil, water, and various power draining accessories to also normalize, and yeah, I expect to see a significant difference between the usually short-hauls, and the longer or more heavily loaded runs...
Even on my older past gasser vehicles, I could always see a measurable difference in engine smoothness and available power after a long drive, as compared to the usual jaunts...
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