There's no way a conventional folding bike can beat a full-sized mountain bike in terms of performance but at least you won't have problems with storage and transportation. To get the best of both worlds, consider a full-sized folding mountain bike like the Montague Paratrooper. It is designed to drop with an airborne soldier, hit the ground, and still meet the demands of rough terrain a soldier is likely to encounter. It takes less than 30 seconds to fold without tools. If it's good enough for the military, it's probably good enough for you.