1. A computer-implemented method comprising: as implemented by one or more computing devices of a content delivery network (CDN) service, the one or more computing devices configured with specific executable instructions, obtaining a Domain Name System (DNS) query from a client computing device at a first DNS server, wherein the DNS query corresponds to a requested resource and wherein the first DNS server corresponds to the CDN service; determining a class of the client computing device associated with the DNS query; and monitoring performance associated with delivery of the requested resource; determining whether an update to routing information for the class is needed based on the delivery performance; and if so, modifying the routing information for the class.