You don't even need to use the Steamworks SDK (see this FAQ), but it's there for you to enhance your game if you do.
Even then, in Sphere under it's software driver, it never reaches 7fps on large maps. So something tells me that they are using an highly un-optimized high level language like Ruby to do the drawing entirely in software. I couldn't see C/C++ being that slow, unless their algorithms are just plain bad. (No tile caching, animation caching, viewport clipping etc.)