This must be something specific to your setup. The fact that it is stable taking 30 seconds strongly indicates this is a timeout - something that is hanging your web server, and it is set to timeout after 30 seconds.
This could be a DNS issue, for example, it’s trying to resolve some address, or contact some external site for some reason.
Maybe you can try scanning the connections on your server with netstat during a request… or check nginx error log, or syslog for additional clues. Good luck!