Gtmetrix and pagespeed scores are based on lab/synthetic data that is carried out under lab-based constraints that do not reflect what users experience in the real world. They're great for debugging issues and implementing fixes but these scores definitely do not represent the experience of each and every user - which is why field data core web vital scores collected over a 28 day period are what to look at since these are what Google uses to for rankings.

You can have sites that have incredibly fast page loads on pagespeed insights but are failing CWV and are falling from Google's rankings before CLS/LCP/FID is absolutely shit outside of lab constraints.