Dashing takes into account both positive and negative speed modifiers. For instance, if our human's movement is currently magically enhanced to 60ft per turn by some rather nice Boots of Speed, it would mean could be able to move a staggering 120ft in a single turn while dashing. This works the other way around too, so if our human took off those magic boots, put some rocks in their backpack and encumbered themselves with minus 10ft movement penalty to slow them down to 20ft of movement, our Dash would only give us 40ft of, not doubling from the original 30ft base movement speed.