The DNS server does not try the second forwarder if the first forwarder does not respond.
If the forwarder responds a SERVFAIL error, then the recursive resolver waits for recursion time-out period before responding back the same to the client.
It takes a long time (about 10 minutes) to load of large number of zones (40,000 to 50,000 zones). This is optimized to load in few seconds (about 15 seconds) now.
See DNS Server Startup Time Improvements in Windows Server 2012 R2 for details.
If the hotfix is available for download, there is a "Hotfix Download Available" section at the top of this Knowledge Base article. If this section does not appear, submit a request to Microsoft Customer Service and Support to obtain the hotfix.
Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft website: Note The "Hotfix Download Available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.