War, even modern ones rarely utilise all the speed that the individual vehicles are capable of.
If you tell a mechanized platoon to make a vehicle march of 200km with good roads, no interdiction, pre recced route and with no breakdowns, wrong turns or accidents along the way it will take them around 4-5 hours to get there stopping probably once for a stretcher and some fuel (never enter a possible combat zone with low fuel). Now delegating those orders from a higher echelon HQ, getting everything in-place, making sure that the route is clean, getting the orders out to the unit, briefing and loading and then collecting said unit upon arrival will add at least an hour to the calculation, possibly two depending on their proficiency.
If you do the same with a battalion it will add several hours as you'll need communications during march and the logistic and CnC will need to be involved and coordinate along the way. If you increase it to a brigade it's quite possible that you'll have a trafficjam and once the first vehicle throws a track or a tyre, takes a wrong turn or forget to pull down their antennas while passing a powerline you're looking at a whole day before you're in-place and combat efficient.
Go up to Division and the problems just keep growing.
Throw some observed possible enemies into the mix, a mined road, roads too weak to carry all your divisional components, to few parallel roads, a blown bridge or viaduct, some thorough hostile who's taken down or flipped roadsigns on their way back, a map error, bad weather and a Bat.CO who doesn't keep coordination with the units behind him and you're looking at considerable slower speeds still. And that's without even getting into any serious combat.
Now wind back the clock some 70 years, take away most of the paved roads, take away good C3 and even up-to-date maps, mechanical tech and materials from the 30's (+50% of the tanks developing some sort of mechanical problem on such a leg) and add some actual enemy resistance and what Guderian, Rommel, von Bock, Patton, Rybalko and other accomplished was nothing short of phenomenal.
Around 3-4km/h on average was not something to be ashamed of for armored forces even though they could leap some 10-20km in just an hour or two and then stop for several more because of X.
Infantry not in direct combat could keep about the same speed but of course once there was a breakthrough the armored formations sped along and then the problems with Corps or Army cohesion started. Not to mentioned that the poor infantrymen were dead on their feet after marching for 10 hours straight in full combat-gear day after day.
HOI3 does a decent enough job to simulate this but for the wrong reasons IMO. I hope we'll see a historically correct but yet immersive and graspable representation in HOI4.
