Tag: Crux

Good little case study on how YouTube optimized their First Contentful Paint and Largest Contentful Paint by applying preload and fetchpriority to their poster image.

My favorite nugget is that they tested using an actual video thumbnail for their poster image versus a solid black poster image, and the black image performed better in user studies:

Using a solid black poster image showed the best results in user studies. Users found the transition from solid black to the first frame of the video to be a less-jarring experience for autoplay videos.

Philip takes a trip down memory lane with some fun stories from the early Yahoo! days around performance.

But most importantly, he suggests that the SEO carrot has tipped the focus of performance, and not for the better.

Sites that truly care about performance and the business impact of that performance, worked hard to make their sites faster.

This changed when Google started using speed as a ranking signal.

I made a similar point in a revamped version of my “Performance Budgets that Stick” talk the other week. If we want this burst of performance interest to stick, and to have the impact we want it to have for users, we’re going to need to make it easier for folks to connect the dots between business metrics and performance.

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.