> The implementation saturates CPU before reaching I/O limits

So, I did look over the code and the thing that I walked away asking was "isn't this sort of the reason why sendfile(2) was developed?"

Axon generates dynamic multipart responses with boundaries and headers to bundle files specified via query parameters. sendfile handles "serve this specific file" but does not handle "bundle these N files into a multipart response."

For static file serving, sendfile would be the better choice.