The name lookup part is strange yes, but it was staying between 400ms to 600ms regardless of that until I made the optimisations.

I found one of the most common reasons for RPCs being slow, was too many connections. I saw some IPs making 100s of connections regardless of the nginx rate limiting. I also saw an ungodly amount of TIME_WAIT (waiting to close) connections that were not being cleaned up.

I copied the sendfile on part from our Privex config, I was a little confused as to why that was there too, but I just left it there as it didn't seem to be hurting anything.

If you take a look at the graphs, you can see the insane open connections that my RPC node, and the minnowsupport one were suffering from. This is partly due to asshats using get_block over HTTP rather than websockets, thus opening 100s of connections (of which by default linux takes 4 minutes to close... which is why time_wait optimisation is needed). This does slow down public RPC nodes due to the fact the network scheduler is having to deal with 2000 connections despite the fact less than 300 of those are actually active.