Speeding wordpress site

Hi, everyone i have little problem.I host few sites but thoes site witch are in wordpress like this http://www.salespc.net/ load very slowly.
I enable varnish and mod_speed and still have slow speed, in gtmetrix before i enable mod_speed and varnish they has more bad score, but no still sites work very slowly.After enable varnish sometimes shows: error 503 backend fetch failed.
CWP is on VM with I3-3*** 12GB RAM, 1 Gbit network.
Can you give me some advice for speeding up sites?

All site are in ssd and vm also.

Looking at the site you gave an example of,

This file is taking 8 seconds to load, because of "Waiting for First Byte (TTFB)"

http://www.salespc.net/wp-content/cache/wpfc-minified/f25d01c2142069b3da6f3fa187ee2054/1520588456index.css

Try resolving that. Perhaps use a different cache plugin in wordpress, such as W3 total cache, Super cache, WP Rocket, keyCDN cache, etc.. So many to choose from but try different ones, check with GT metrix and compare results.

That would be the first thing to do.