Every zone has a copy, and clients always read their local zone copy (via pooled memcached connections) first and fallback only once to another zone on miss. Key is staying in zone and memcached protocol plus super fast server latencies. It's been a little while since we measured, but memcached has a time to first byte of around 10us and then scales sublinearly with payload size [1]. Single zone latency is variable but generally between 150 and 250us roundtrip, cross AZ is terrible at up to a millisecond [2].

So you put 200us network with 30us response time and get about 250us average latency. Of course the P99 tail is closer to a millisecond and you have to do things like hedges to fight things like the hard coded eternity 200ms TCP packet retry timer ... But that's a whole other can of worms to talk about.

[1] https://github.com/Netflix-Skunkworks/service-capacity-model...

[2] https://jolynch.github.io/pdf/wlllb-apachecon-2022.pdf