For some reason all the POST request are delayed for about 1 min. Here is my configuration:

/etc/nginx/nginx.conf

sendfile on;
tcp_nopush on;
tcp_nodelay off;
keepalive_timeout 65;
types_hash_max_size 2048;
proxy_buffering off;
proxy_http_version 1.1;
proxy_set_header Connection "";

/etc/nginx/sites-available/default

client_max_body_size 0;
send_timeout 300;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header Host $http_host;

location / {
# First attempt to serve request as file, then
# as directory, then fall back to displaying a 404.
try_files $uri $uri/ /index.php?$query_string;
}

/etc/php/7.2/fpm/pool.d/www.conf

pm = ondemand
pm.max_children = 60
pm.start_servers = 20
pm.min_spare_servers = 20
pm.max_spare_servers = 60