There are multiple websites and toolings that help you measure your perfomance score. Three of the best known are Pingdom Tools, GTmetrix and Google's Pagespeed Insights. When you are auditing your website, which one should you use?
Technical checklist vs realtime mobile data
I consider Pingdom Tools and GTmetrix as practical technical checklists (with some anti-patterns / nuance as well, but that's another story), where pagespeed metrics such as onload or DOMContentLoaded alone unfortunately aren't telling you the whole UX story. Additionally, the default testing with Pingdom Tools and GTmetrix is done using a desktop device. Due to CPU boundaries/differences, this is not representative for mobile users.
- one could do a better job towards user engagement when using critical CSS, while DOMContentLoaded could still be bad, but at least UX is reasonable;
- or be doing a worse job when using spinner/splash screens, having a fast DOMContentLoaded, but users won't see anything meaningful soon, meaning no early user engagement.
That's where the UX minded metrics of PageSpeed Insights (and for example Lighthouse, Webpagetest.org) come in handy! Especially as Google's new WebVitals is all about (technical) UX as well.
🚀 Happy (online) speeding!