Giving different speeds per terrain type would be possible, however, the new pathfinder wouldn't be able to handle it. It was foreseen in the old pathfinder, but as everyone knows, that's too slow and had more serious problems too. So certain features had to be taken out of it, the main one is different speed according to terrain.
This means that your units won't follow the fast terrain, but will still go in straight lines over different terrains. This will cause more micro (people trying to avoid slow terrain), and thus is something we want to avoid.