DNS Session Delays
The DNS session delay exceeds the critical value set in Expert Thresholds. The "session delay" metric combines network latency (i.e., the time spent waiting for data to traverse the network) and application processing time (i.e., the time spent waiting for the server or application to respond to a request). DNS Application Processing Time for DNS is tracked with a separate Expert Threshold.
Possible reasons for the event:
There is a hardware or software problem at the application end, which you can verify by checking Application Processing Time. Perhaps the DNS server hardware is overloaded with disk I/0 requests, or some other system component (for example, the CPU or NIC) is too slow given the traffic it is expected to handle.
Excessive latency caused by network congestion or misconfiguration. You should look for problems with the network, focusing on UDP performance. Do you have enough bandwidth between the application server and the clients that use it? Are the routers between the systems configured correctly? Are the Maximum Transfer Units (MTUs) along the path optimized and compatible?