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Unfortunately the Linux DNS resolver has no direct support for detecting and doing failovers for DNS servers. It keeps feeding requests to your primary resolving nameserver, waits for a configured timeout, attempts again, and only then tries the second nameserver.
This typically means nearly 30s delay for all request as long as your primary nameserver is unreachable. It doesn't learn to directly target your secondary nameserver so long as there is trouble.
Even with the most optimal configuration, the delay will still be measured in seconds per request. For many requests, that's many more seconds.
I wanted to solve this.