Magento 2 Slow Product Saving – How to Fix Slow Product Saving Issue

After compared with the /etc/varnish/default.vcl, I found after removing the .probe section in the backend default can solve this. Still wondering why the probe caused this

backend default {
    .host = "127.0.0.1";
    .port = "8080";
    .first_byte_timeout = 600s;
    .probe = {
        .url = "/pub/health_check.php";
        .timeout = 2s;
        .interval = 5s;
        .window = 10;
        .threshold = 5;
   }
}

to

backend default {
    .host = "127.0.0.1";
    .port = "8080";
    .first_byte_timeout = 600s;
}

Updated:

Just change the following line:

.url = "/pub/health_check.php";

to

.url = "/health_check.php";

solved the problem, refer to this article https://www.sohaib.com/magento-2-12-varnish-error-503-backend-fetch-failed-solved-php-7-1-nginx-varnish-5-x/

this has been resolved, it was a combination of the /pub/health_check.php issue seen elsewhere, missing virtualhosts as a result of the server being locked to only port 443 for security (we made a new vhost for port 80 for varnish, and for port 8080 for apache's backend listen port) AND the DocumentRoot line not being required in the virtualhosts file for the SSL connections (but IS required in the vhosts for port 80 and 8080)

hope this helps anyone else with our issue to resolve it, the site is flying now.