I have two bind9 servers that are not setup with duplicate settings and know nothing about each other. Our CPU usage is sitting around 7% - 10%: mostly bind, which I believe is perfectly in range for our load. However, CPU usage is slowly growing increasing every week and I estimate that I have at least 4-5 months before this becomes an issue. Just want a clarification - would a Master/Slave setup reduce CPU on the slave?
We are using the service as a forwarding server allowing IP ACL based recursive look-ups. The only fail over is the fact that our DNS users input both DNS addresses in their setup. It doesn't do dynamic updates, we have a small list of DNS records that get changed maybe once or twice a quarter.
DNS operates best in a Master/Slave scenario. Consider this: you have two nameservers, ns1 and ns2. To help balance the load, you have two NS records in your domain record for both ns1 and ns2. By the very nature of DNS, clients will query the nameservers in random order because nameserver lookups are returned in a random order. See this answer for helpful information.
If you're running into performance problems even with this current setup, add another nameserver to help support the load or give your existing servers additional resources.