Reminds of a developer I hired through Fiverr that claimed he could achieve 99/100 Google pagespeed score for any Shopify website (and improve overall load times of Shopify). He implemented his changes, and sure enough it scored 99/100. However, the site still seemed to be running slowly when I tested it. I dug into what code he had actually changed, and there were only a few lines of JavaScript injected into the site. The JS was obfuscated, after de-obfuscating it I realized all his code did was prevent any other JavaScript from being loaded if it detected the user-agent was coming from Google pagespeed :)

After some back and forth with Fiverr I was able to get refunded, but I'm sure the thousands of other customers that gave him 5 star reviews have no idea.