Maybe I'm missing something, but... In "Year of 4 emperors" I have in spring 2 large stacks under Valens & Caecina trying to cross the Alps to Galia Cisalpina through 2 different mountain passes. If I keep in these stacks Germanic mercenaries (some of heavy infantry, cavalry and archers from a starting set-up) predicted times to pass the Alps are like 60-80 days. Crazy. If I remove these Germans, leaving in stacks only legions, but also Gallic auxiliaries, their time suddenly is like 16-18 days. WTH? The difference has no rational explanation to me.
BTW, freezing rivers offer interesting tactical possibilities. But in my current campaign I have a small remains of beaten Otho's army from Vindonissa, which probably try to run after the siege through then frozen river, but somehow stuck on it (Rhenus Superior) and now they sit on a river (so says toolip) - can't be attacked by ships, by land units, are not moving themselves. Apparently a bug.