NGINX waiting 60 seconds before processing request for NodeJS application

Question

I have a NodeJS (ExpressJS) application containerized on Docker that is exposed through an NGINX reverse proxy also containerized on Docker. Sometimes, a request sent to NGINX takes 60 seconds before being sent to the application. In my monitoring on Sentry, I see that the server responded to the request in 600ms, but on the browser the download time is 61 seconds.

Here is my Nginx configuration

worker_processes  1;
error_log /dev/stderr warn;

events {
    worker_connections 1024;
}

# make sure to set plaintext JWT_SECRET environment variable
env JWT_SECRET;

http {
    log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';
    access_log /dev/stdout main;
    lua_package_path "/usr/local/openresty/lualib/?.lua;;";

    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_prefer_server_ciphers on;
    ssl_ciphers "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384 EECDH+ECDSA+SHA256 EECDH+aRSA+SHA384 EECDH+aRSA+SHA256 EECDH+aRSA+RC4 EECDH EDH+aRSA RC4 !aNULL !eNULL !LOW !3DES !MD5 !EXP !PSK !SRP !DSS +RC4 RC4";

    gzip on;
    gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
    # API
    upstream application_server {
        server application_server _1:3000 max_fails=1 fail_timeout=5s;
        server application_server _8:3000 max_fails=1 fail_timeout=5s;
        server application_server _2:3000 max_fails=1 fail_timeout=5s;
        server application_server _3:3000 max_fails=1 fail_timeout=5s;
        server application_server _4:3000 max_fails=1 fail_timeout=5s;
        server application_server _5:3000 max_fails=1 fail_timeout=5s;
        server application_server _6:3000 max_fails=1 fail_timeout=5s;
        server application_server _7:3000 max_fails=1 fail_timeout=5s;
    }

    server {
        listen 443 ssl http2;
        server_name api.application.dev;
        client_max_body_size    200M;
        ssl_certificate /etc/letsencrypt/live/api.application.dev/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/api.application.dev/privkey.pem;

        location / {
            expires -1;
            proxy_set_header    Host $host;
            proxy_set_header    X-Real-IP $remote_addr;
            proxy_set_header    X-Forwarded-Proto   https;
            proxy_set_header    X-Forwarded-For $remote_addr;
            proxy_set_header    X-Forwarded-Host    $remote_addr;
            proxy_pass http://application_server;
        }
    }
}

What can I do to track more this issue and fix it?