The response time check measures the average time for the name server to respond to a query.
DNSmonitor KB - DNS check KB

Response time check

The response time check measures the average time for the name server to respond to a query.

The response time is measured over five query attempts and the average time for these five transmissions is calculated. Time-outs will be ignored and does not affect the response time.

The check does not return any messages.

Since most authoritative name servers keep a copy of the zone file in its memory there is little or no difference in response time depending on which type of query used. In this check a SOA query is used mainly because that record is always present. The response time usually varies between locations which is natural.