Why is the routing and fare calculation a big problem? I can see writing to a legacy system being slow, but I would think there could be constant caching somew

Why is the routing and fare calculation a big problem? I can see writing to a legacy system being slow, but I would think there could be constant caching somewhere of the data as it is read by anyone, and thus reads could be mostly fast, making routing calculation fast. Then it would be a matter of checking for validity of chosen routes.