Tag: Http Archive

This post summarizes how we analyzed publicly available web transparency data and ad hoc A/B testing to understand the adoption and performance characteristics of native image lazy-loading. What we found is that lazy-loading can be an amazingly effective tool for reducing unneeded image bytes, but overuse can negatively affect performance. Concretely, our analysis shows that more eagerly loading images within the initial viewport—while liberally lazy-loading the rest—can give us the best of both worlds: fewer bytes loaded and improved Core Web Vitals.

Surprisingly, the correlations for each CWV are weak to medium strength and for FID it’s actually a negative correlation, meaning that as the Lighthouse score goes up, real-user FID performance actually tends to go down a bit.

This is the kind of analysis I was hoping to see after Pat added CrUX data to WebPageTest.

Most of this lines up with what I’d expect. Cumulative Layout Shift is measured very differently synthetically versus in the CrUX data (particularly before the new windowing approach) and First Input Delay has always seemed to have a very weak connection to Total Blocking Time in my experience. (First Input Delay itself has plenty of limitations, and I’m eager to see it supplanted by something a bit more useful in the future.)

I think many of us have cautioned against leaning too hard on optimizing for your Lighthouse scores, and it’s nice to evidence as to why. Lighthouse is a great tool, but it works better as a “here’s a list of things you can try to improve” than it does as a goal in and of itself.