I have found that "gzip on" setting for the nginx webserver is counter-intuitive, in that it drastically slows page loads when the performance settings of BD are set. Double-compression?
ISPConfig sets gzip on by default. 

If anyone else can confirm the issue, it would be good to have the docs modified here:
https://docs.backdropcms.org/documentation/system-requirements#web-server-details
 

Cheers!

Comments

If I recall correctly, Backdrop will not gzip if the page content is already gzipped. And it only happens if you've enabled page compression setting. 

But maybe that doesn't help here where I'm guessing this particular nginx config is attempting to gzip and isn't checking the current page. But hard to know without access.