Just for giggles, I played the third tutorial after victory. I had all of my forces in Cotebus and wanted to move on Berlin, A major road leads to Berlin but, when I ordered everybody with synchronized movement to Berlin, the movement went through Luckenwald and took 20 days. Was I putting too many men on that road?
I 've encountered this too. When you capture a fort, you get a unit of fortess artillery that is automatically attached to your corps which will slow movement considerably. Detaching the unit will speed things up.