An Accept-Encoding header is an HTTP header that is used to tell the client/browser if it can handle compressed versions of web page content. It must be included on every origin server response.
Specify a Vary: Accept-Encoding Header (Deprecated)
What is the Accept-Encoding Header?
Why is the Accept-Encoding Header Necessary?
The message itself can appear is situations where a CDN delivers content to a browser that does not support compression. When this happens, the server ends up delivering the uncompressed content, which is inefficient and will cause the performance to be slow. Having this header set correctly ensures that the appropriate content is always delivered correctly and as quickly as possible. The good part is that a lot CDN service providers, and even WordPress plugins, set this automatically, which is why this optimization is technically deprecated. However, it is good to know in advance in case you ever come across this warning message.
Lighthouse is an open-source tool that is used to run an audit against your web pages and provides scores and suggestions for improving page performance, accessibility, SEO, and more.
PageSpeed Insights is a tool created by Google that reports on the performance of a page on both mobile and desktop devices, and provides suggestions on how that page may be improved.