nginx slow symlink subfolder

I have two domains, like this:

1. www.main.com in /var/www/www.main.com (without /public)
2. system.main.com in /var/www/system.main.com (with /public)

In the first one I have a subfolder www.main.com/sub that sym-links to the second domain.

/var/www/www.main.com/sub => /var/www/system.main.com/public

To make PHP (FPM) work in the /sub I used this config:

    location /sub {
            try_files /sub/$uri /sub/$uri/ /sub/index.php?q=$uri&$args ;
    }

That works fine but is very slow.

Simple request on www.main.com/sub takes 600ms, while the same request on second domain directly takes 100ms.

Does anyone knows why this is slow and how to fix it?

What I tried

Following this I tried including the other project root and PHP more direct but could not get it working:

location /sub {
    alias /var/www/system.main.com/public;
    index index.php;
    try_files  $uri $uri/ /index.php?$args ;

    #if (!-e $request_filename) { rewrite ^ /index.php last; }

    location ~ \.php$ {
                include /etc/nginx/fastcgi_params;
                fastcgi_pass unix:/var/lib/php7.0-fpm/web8.sock;
                fastcgi_index index.php;
                fastcgi_param SCRIPT_FILENAME $request_filename$fastcgi_script_name;
                fastcgi_param DOCUMENT_ROOT $realpath_root;
                fastcgi_intercept_errors on;
    }
}

Update

I got it working for index.php but still have trouble with any path.