Checking a certain site's page (example.com/page.html) using for example, PageSpeed, shows issues with compression:
Enable compression for the following resources to reduce their transfer size by 18.3KiB (64% reduction). Compressing https://ex.co/sitepage.js could save 18.3KiB (64% reduction).
Gzip compression is setup correctly on example.com:
# grep 'gzip_types' /var/www/vhosts/system/example.com/conf/vhost_nginx.conf
According to PageSpeed check, it does not validate https://ex.co/sitepage.js as compressed. example.com/page.html contains a link to a third-party file (located on a different site): https://ex.co/sitepage.js. Gzip compression is not enabled on ex.co site.
Use one of the following solutions:
1. Contact ex.co site owners in order to enable gzip compression. If this site is hosted in Plesk, the following article should be followed:
2. Change example.com/page.html page code so that it does not include links to a third-party sources. Instead, all Java files should be stored locally under example.com