I'm glad that I was able to help you. I am not a domain admin nor would I call myself a expert on the inner workings of SMB filesharing, but I'll explain how I came about knowing this.
About 2 years ago a coworker of mine posed the same question that you had and provided a packet capture for me. Before I could even review the packet capture they had wiped out the machines settings and when reconfigured, without DNS, they noticed that the problem went away. I never really dug into the pcap because this was fine for the customer, problelm solved.
Late last year another coworker, in another state, escalated the same issue on the newer AltaLink machines and provided a packet capture.
I know enough about packet analysis to find out what the root cause of a problem is most of the time, so I had a look at those 2 packet captures side by side. What I saw in them was identical. Repeated reverse DNS lookups that continually failed causing scans to deliver slowly.
That led me to start gaining knowledge on SMB, reverse dns, reverse dns lookup zones, and PTR records. So rather than recommending that this new customer remove DNS servers; I recommended that they properly configure a reverse DNS zone and accompanying PTR record.
The Analyst that started the escalation closed the case the following day stating that the domain admin configured reverse DNS and PTR properly and the problem was resolved.
If you are interested in learning more through personal analysis you can take a packet capture using the built-in network troubleshooting log function found in the security section of the Xerox Embedded Web Server. Take a capture of the scanning process with your previous DNS servers configured and one with the new ones and take a look at the difference in Wireshark.
