That is a big hint to why you feel you're going slow. On flat or downhill ground I would usually be on the very hardest gear in order to be able to keep up with the wheels. If you're constantly in a medium gear you'll not reach very high speeds on easy terrain.

But maybe you shouldn't worry about that right now. You're pretty new to this, so you don't need to optimize for speed. Just go at a pace comfortable for you, while you get used to this new way of moving. Perhaps it's even a good thing to be going slower right now, that way there's more time to react, and less injury if you have an accident.