Hold down shift to make slopes (lets you aim up/down on a slope to sort of smooth it relative to where you stand). The flattening only goes a bit at a time, and not too much from where it's starting. You'll need a pick to lower heights that are too high, and you'll need rocks to fill where it's too low, eventually. You can only change the terrain something like 10 height tiles or the like (maybe 15?) -- so no caring a flat path through a mountain or anything -- but you can cut a road.

And nope... there's no speed up unless you mod. It's a labor. It's supposed to be. This is a pretty chill game that expects you to put in the labor. If it feels slow, go try out Wurm Online/Unlimited for a bit and you'll come back thinking this game is super fast.