If you tell nginx to limit itself to 4 workers x 1024 connections per worker = 4096 connections, and hurl 10k connections at it, of course it's going to throw e

If you tell nginx to limit itself to 4 workers x 1024 connections per worker = 4096 connections, and hurl 10k connections at it, of course it's going to throw errors. It's doing exactly what you told it to do. That's just one example of how OP's "optimized" nginx config is barely even optimized. There are lots of other variables that you can tweak to get even better performance and blow Caddy out the window, but those tweaks are going to depend on the specific workload you expect to handle. There isn't a single, perfectly optimized, set of values that's going to work for everyone. The beauty of Caddy is that you get most of that performance without having to tweak anything.