This kind of warning is very common in Speed Optimization. This is actually an HTTP header what is responses from the server to the browser. This warning shows if your server is not Vary header is not configured.
What is Specify a Vary: Accept-Encoding Header
When a browser makes a request, they include HTTP headers for the server to decide what server should response. The Vary header describes what information identifies inside of that HTTP header (what is the request). Caches should only be used if the request matches the Vary information in the cache.
Fix “Specify a Vary: Accept-Encoding Header” Warning
You can only fix the vary headers what is in your server. You cannot fix the 3rd party VARY HEADERS. Fixes of VARY headers depends on the servers. So it differs fixes between Apache and Nginx.
Fixing “Specify a Vary: Accept-Encoding Header” Warning in Apache
If you don’t have root access of your server open your .htaccess and add these following lines of codes:
Header append Vary: Accept-Encoding
Fixing “Specify a Vary: Accept-Encoding Header” Warning in Nginx
To fix Specify a Vary: Accept-Encoding Header in Nginx you need to open
Using the Vary: Accept-Encoding header could decrease the speed of your WordPress site and it will ensure you are using gzip compression.