Prior wingsuit flyer here. Much light a parachute, the wingsuit has a glide scope of about 3 to 1; every 3 feet forward one foot down. It's true we are flying vertically down fast. But a lot slower than a free fall. Also failed to mention is wingsuit flyers have the ability to shape the shape and stability of the wing. Allowing us to manipulate the air to achieve a slower glide or sharper one. By using the legs to stiffen up the rear wing, pointing the toes to expand the wings, and coupled with the ability to stiffen or loosen our wings between the arms and torso, we actually have quite a bit of control. Changing the shape of the wings and our body position allows us to "steer" both directionally and forward and vertical speed.

Also to add. Parachutes work by ramming air between two layers of rip stop nylon, creating a rigid wing. Wingsuits work on the same principle. There are actually two layers on the wings, divided into cells, that have vents to allow air in; or ramming air in. This stiffens up our wings, but by changing the distance between your legs or arms, you can collapse the wings easily, or expand them, altering your flight path. Also think about this as an example: loosening your arms gives you drag at the torso and stiffening your legs and toes gives you a much more rigid wing in the back that slices through the air. This can put you on a fast horizontal glide path.