The objects displayed in a waterfall chart that have no DNS Lookup times are objects that were downloaded during a previously established connection.
DNS Lookup refers to the time it takes to translate the host name (e.g., www.dynatrace.com) into an IP address (e.g., 220.127.116.11).
The DNS Lookup occurs for every connection that the Agent makes when executing a test script. Several objects can be downloaded from a server without making additional connections and executing new DNS Lookups. Downloading several objects through one connection lowers the overall response time for downloading the page.