No, it doesn't slow troops march. Only at brigade level you'll notice that troops wait for start to move one behind the other.
Anyway I usually try to not put too many troops on the same road because you could find enemy closer than what you expect and then you could find very difficult to deploy all your men. You can also find more difficult to manage and keep control over your subordinate.
If I want to use the same road for all my forces, once the first is moving, I delay the move orders for the second, the third and so on.