Tuning NPM/NGINX performance

Maybe not an issue, but I've tried benchmarking NPM using loader.io, with 1000 users/s it has around 22000 success rate, while directly stress test the web server (nginx) I can get around 56000 success rate.
I tried edit nginx configuration inside the container (npm_app_1) but still no luck. 
Any help would be greatly appreciated! Thanks