GTmetrix is a tool for analysing you (or your clients) website for components that slow down the delivery of the code to the visitors browser.
It gives you a breakdown of the exact files which are causing problems, and most the time a fix for them. A quick Google search for the other problems revealed the other fixes (Some do require intermediate skills to remedy). GTmetrix helped speed up delivery of a wordpress from 55% to 88% on one of my own projects; Giving visitors a much better experience when loading the site for the first time.
I used this to identify problems on this site, and it helped reduce image load times massively. It also helped find problems with the nginx on the server itself, and was able to find fixes via google.
When you need better SEO results and visitor experience for your website
Some errors remain, and getting a 100% doesn’t seem possible without paying them to help