Take a closer look, on normal terrain the infantry already seem to slide a little even when just marching. It's only when they're on snow that the speed seems "normal", but still a tad too fast IMO. I think the snow default speed minus ~15% should be the default speed for grass, while the present snow speed should be even slower.

Of course, just my opinion... but really look closely! It's like they're on fast forward especially the cavalry!