I have spent the day troubleshooting why one server in our network
/ tested configuration extremely slowly.
We have found that server_names scales very poorly, once a certain point
reached (approx 5.5k entries globally, 5k entries for a single host)
performance drops from a <0.5s reload time to 15s+.
The large host of ~5,000 entries is a malware domain zone and all server
names in this zone are using the wildcard name format.
For now we have resolved this issue by fixing an inefficiency in our
configuration (namely using *.domain.com and domain.com) however I feel
this is most likely a bug or at-least an unintended behaviour.
Relevant configuration entries: