I had an idea, need to see if is doable.

Not taking into account other problems the game may have, For me, one glaring issue of this game, that would help re-doing campaign too, is that the maps in campaign (IDK in endgame yet, but i played most char in act 1, currently doing final lap of act 2 with witch) are too big.
I think, one of the solutions to this would be to have a MS buff per % of mobs in the map killed.

For example (Don't know correct numbers, but to be clear): If you killed 25% of the mobs of that map, you get 10% ms bonus. 50% you get 15%, 75% you get 20%. You get the gist.

This would make that clearing the map is still good, keeps the "killing and exploring" theme going, you can go to the side you didn't explore faster, and doesn't mean you are not killing anything and zooming over the map. Just a thought, don't know if this is the appropiate forum thingy to do it.

Horrible take