I honestly don't know, I never made serious performance tests. The external exporter can make use of every variable that nginx is able to use in a log_format. I

I honestly don't know, I never made serious performance tests. The external exporter can make use of every variable that nginx is able to use in a log_format. I guess the internal Lua scripting has access to even more data. Performance wise I assume the external exporter has less impact on Nginx' operation because it doesn't keep shared state or run code inside Nginx at all (see https://github.com/knyar/nginx-lua-prometheus#caveats https://github.com/knyar/nginx-lua-prometheus#caveats), all Ngnix does is push out log records over syslog/UDP. If the exporter should overload that doesn't concern Nginx at all, what happens is that observations get dropped.