McClellan seemed to be a half empty glass of water person. He saw obstacles. When finding the Lost Orders, he ordered Franklin to move "at daybreak in the morning." His orders were clear and precise but nowhere in them is there any hint of urgency. To march "at daylght" was good but to march upon reciept of these orders would have been better. Franklin's VI Corps was well-rested having not seen fighting since the Peninsula. The roads were good, the weather clear, and a night march was perfectly feasible. This way Franklin could have struck the gap at South Mt at first light.
Well, the Jefferson Pass wasn't seized until near nightfall by cavalry and elements of the 6th and 9th Corps; the fact that 6th Corps elements were involved with taking the pass indicates in and of itself that 6th Corps was not wholly stationary. Given that McClellan sent the order to Franklin after sunset (6:20 PM) and they were on the eastern side of a mountain range it was probably already quite dark; given travel times Franklin would have recieved the order probably after 7PM, at which point you're dealing with trying to orient and march troops when it's too dark to point them in the right direction.
A reference I have (an article by Gene Thorp) states that part of 6th Corps did a night march to Jefferson pass.
In the case of the pass at Braddock Heights (which was secured in the middle of the day and which has the National Road running down it) McClellan has most of the 9th Corps advance overnight with 1st Corps behind. This wing of the army
does attack not long after dawn (8AM) though then have to fight all day, while Franklin reached Burkittsville around noon and takes another three hours to close up, though how much of that distance was made at night is hard to determine for sure.
Cox left Frederick around 3PM and the attack began around 8AM; that's the results of about five hours of daylight marching, plus a couple of hours of twilight marching, plus whatever was done at night. Franklin left the foot of the Catoctins around dawn and reached Burkittsville around noon, which is the result of about six hours of daylight marching, plus up to an hour of twilight marching.
The amount of distance Cox and the 9th Corps made at night might not have been very large in a real sense. At that, 9th Corps has the National Road to use as a guideline - it's comparatively easy to tell you might be making a wrong turn if you're no longer on a specific good quality road, while the road network around Jefferson does include a good quality road but it's not the one that 6th Corps is supposed to take. A wrong turn by night could leave 6th Corps hopelessly confused, which is a reason why retreat movements by night are more common than advances to contact by night. (As it happens, Franklin's corps had done almost exactly this after Glendale, and without orders to boot - they'd gone some miles down the wrong road by night.)
Of course, there might have been consequences for running 9th Corps ragged - they were slow for the next few days.
McClellan could drive his men just a hard as he was driven, but no harder. He never asked them to achieve more.
I mean, McClellan's movements in Maryland
are fairly rapid, and he's the one arguing to advance against the enemy afterwards. During the Loudoun Valley campaign some of his corps march upwards of fifteen miles, while back at the start of Yorktown he orders full on attacks by both wings as soon as they arrive.
Having read McClellan's actual order to Franklin, it specifies what Franklin is to do in certain contingency situations, outlines McClellan's goals, orders him explicitly not to wait for Couch, and asks him for (among other things) "the utmost activity that a general can exercise" and to attack half an hour after the main attack on the National Road; as it happens Franklin does in fact wait for Couch at Jefferson (though he then moves on) and waits until he has more troops formed up before going for it rather than attacking as soon as he arrives.
This could be attributed to McClellan not using sufficiently urgent language in his message, but Franklin actually has fewer brigades in his corps than McLaws does in the Pleasant Valley; I think Franklin might really need to get his whole force closed up to
exploit in the Pleasant Valley.
(Franklin's 6th Corps proper, 271 companies; Couch, 144 companies; McLaws and Anderson, 430 companies.)