Performance can mean many things. The time to transfer a file is one measure, while the responsiveness of an interactive remote login session is another. The time to call up a Web page is still another. If an ISP drops many packets, it will clearly take longer to transmit data or do remote operations because packets need to be retransmitted. Network delay is the time it takes for packets to go through a network.
Many applications simply idle while waiting for DNS information.
Adding network delay to DNS query times only makes things worse. There are a number of reasons that packet loss and network delay are more critical than throughput. First, throughput will have an absolute upper limit determined by the size of the connection. Second, measuring throughput requires that you have a significant amount of data to move to or from some Internet system. It is not always possible to have this.