DNS Tip: If you use anycast routing to ensure all client traffic is sent to the opposite side of the world, and all of your DNS servers are as slow as possible, you naturally rate-limit busy clients with no software or hardware changes or increased memory use.

If RTT is 300 ms, for example, one individual client won't send more than 3 requests per second!

This optimization method is widely deployed by major ISPs and authoritative DNS providers.