The BIG League

It was cool seeing both Ant-Man and The Atom grow to giant-size and battle in Captain America: Civil War and DC's Legends of Tomorrow respectively, but I have to admit that I had the same issue with both action sequences.  I'm not sure why when they size up, they suddenly start moving in slow motion.  It seems to undermine any advantage they get from being huge.  Wouldn't they still move around at a normal rate?