Google On How It Measures Web Vitals

Martin Splitt from Google went a bit deeper, on Twitter (yes on Twitter), on how Google goes about rendering and measuring what it renders. Here is what Martin Splitt said on Twitter.

First, here was the question from Victor Pan, "If Googlebot uses a non-googlebot user-agent to check CLS, LCP, and whatever other Web Vital metric, what would its name be called? We implemented dynamic rendering for googlebot cc @methode @JohnMu - should those dynamically rendering worry about Poor URL web vitals in GSC?"

I'll embed Martin's response:

Here's my lenghty answer :) Grab a seat, make yourself at home! Let's dissect this question into:

- Googlebot measuring things
- Dynamic rendering and performance
- Performance and UX
- rUM vs. lab data
- GSC Web Vitals report

Off we go!

— Martin Splitt @ 🏡🇨🇭 (@g33konaut) August 4, 2020

2/? When you use dynamic rendering, you may incur a bit of a delay while the renderer is busy producing the static HTML. You can work around that with caching and pre-warming the cache regularly. But the performance that Googlebot sees vs. what the user sees would be v. different

— Martin Splitt @ 🏡🇨🇭 (@g33konaut) August 4, 2020

4/? So that brings us to measuring things.
Googlebot (or Lighthouse, or PageSpeed Insights) measures *lab data*, which is hypothetical performance data in a pretty ideal environment. This isn't representative of what actual users see - but that's more important...

— Martin Splitt @ 🏡🇨🇭 (@g33konaut) August 4, 2020

6/6 And that's the cool thing about the GSC web vitals report - it shows you limited data (b/c not every URL might have enough rUM data) but it's real user metrics! So if something is "poor" there, it means real users suffered. You wanna fix that for sure.

— Martin Splitt @ 🏡🇨🇭 (@g33konaut) August 4, 2020

Hold on, hold on. I got to deep into the implementation details AND misremembered something. Yay 🤦

Page Experience won't use lab data for core web vitals, at least as it's planned at the moment. You also don't need to worry about the details TBH

— Martin Splitt @ 🏡🇨🇭 (@g33konaut) August 5, 2020

Useful details to know and add to your SEO and developer arsenal of knowledge.

Forum discussion at Twitter.