There is, Amethyst will connect to all the relays specified, some user have 20-30-40 relays, that means Amethyst connects to all of them and retrieves each event the user posts that amount of times. Nostria will by default connect to 3-4, is user-configured, and it optimized selection if a user is using a common relay. Setting it to 1, would save even more bandwidth. Plenty of ways to optimize bandwidth usage, though it will never be as good as centralized services.