Hi Steve,
Without looking through ONTAP source code, i'd assume that the SVM ("client machine") uses DNS to locate the domain controller as per the following:
https://msdn.microsoft.com/en-au/library/cc717360.aspx
To locate domain controller (DC) hosting NC N, the client machine issues a DNS query for the SRV record _ldap._tcp.dc._msdcs.N, constructed from the NC name (N).
I don't have details on the status results but i'd also assume that "slow" referres to domain controllers that have been identified using a slow link detection algorthim based on ICMP.
Is there a particular issue you are trying to troubleshoot? If so i'd verfiy the AD sites and services configuration and site network link speeds.
/Matt
If this post resolved your issue, help others by selecting ACCEPT AS SOLUTION or adding a KUDO.