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 is associated with a requested resource and wherein the first DNS server is associated with the CDN service; determining a class of the client computing device associated with the DNS query; and modifying routing information for the class based on performance associated with delivery of the requested resource.