I meant to note, I'm still looking at the scaling & failure rate behaviour. But I'm getting such confusing results that it's hard to even know where to start.
One thing that's increasingly clear is that the wrk results are wildly variable. e.g. if I run:
wrk --threads 20 --duration 30s --connections 4000 --latency --timeout 2s http://127.0.0.1:8080/bench/10000
…sometimes I get nearly 40k req/s and basically no timeouts, and other times I get mere tens of requests per sec, and basically everything times out.
I don't know if the problem is wrk or Vapor or what, but with such absurdly large variations in performance and general behaviour, it's hard to quantify anything.
But this only happens with high numbers of connections - if I lower it to about 3,000 or less, the variability goes away.
(thankfully I was using a mere 20 or 100 connections for my earlier optimisation work - and I did test pretty thoroughly that those results were consistent across many runs)