nginx parllel uploading issue form

I have an php web app that buils on laravel5. i use nginx. in my website there is an contact form where user can upload multiple files . My issue is whenever user uploading multiple files its not uploading instead its showing loading for very long time. if upload single file then its uploading instantly. i'm not sure why exactly

OS: ubuntu 14.04 x64
nginx/1.8.0 , PHP 5.6.14-1

any suggestions ?

nginx.conf :

              `
worker_rlimit_nofile 40000;

http {

##
# Basic Settings
##

proxy_connect_timeout  600s;
proxy_send_timeout  600s;
proxy_read_timeout  600s;
fastcgi_send_timeout 600s;
fastcgi_read_timeout 600s;

sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;
server_tokens off;
client_max_body_size 512m;

server_names_hash_bucket_size 64;
# server_name_in_redirect off;
##
# Gzip Settings
##

    gzip on;
    gzip_disable "MSIE [1-6]\.(?!.*SV1)";

    gzip_vary on;
    gzip_proxied any;

          `

sites-enabled default.conf :

    fastcgi_connect_timeout 140;
    fastcgi_send_timeout 400;
    fastcgi_read_timeout 400;
    fastcgi_buffer_size 128k;
    fastcgi_buffers 4 256k;
    fastcgi_busy_buffers_size 256k;
    fastcgi_temp_file_write_size 256k;

via Chebli Mohamed