> [1] to be honest, I'm not sure I understand the intent of open_file_cache... Opening files is usually not that expensive

I may have a hint here - remember, that Nginx was created in the times of dialup was a thing yet and having single Pentium 3 server was a norm (I believe I've seen myself that wwwXXX machines in the Rambler DCs over that time).

So my a bit educated guess here, that saving every syscal was sorta ultimate goal and it was more efficient in terms of at least latency by that times. You may take a look how Nginx parses http methods (GET/POST) to save operations.

Myself I don't remember seeing large benefits of using open_file_cache, but I likely never did a proper perf test here. Say ensure use of sendfile/buffers/TLS termination made much more influence for me on modern (10-15 years old) HW.