Minimum level flight speed is called an aircraft’s stall speeds, which is different than their ideal glide speeds. Wingsuits have a glide ratio of ~2.5:1 (horizontal:vertical) which is not that much better than just falling as a normal skydiver can hit 1:1.

To maintain level flight they need to generate a lot more lift, while carrying the extra weight of an engine. By comparison a 747 has a glide ratio of 15:1 and depending on specific model and how much weight it has onboard it will stall in level flight around 180MPH. While many aircraft can fly at much lower speeds, a wingsuit is a terrible aircraft.