Why does an Active Directory (AD)-integrated DNS server take longer to start than a typical zone-based DNS server?

A. Windows 2000 and later OSs can store DNS information in AD if the DNS server is a domain controller (DC). Alternatively, the OS can store DNS information on a standard primary zone-based DNS server, which is file based.

When the DNS service starts, it loads all zone information into a memory cache, regardless of whether the OS maintains the DNS information in AD or in a file. DNS information stored in a standard primary zone (i.e., read from a zone file) will load faster than information stored in an AD-integrated zone because the integrated zone must read all its records from AD. This difference in performance is simply an effect of reading information from different media (file versus AD).