An ESSENTIAL concept to understand with speed is the difference between lab data (or synthetic testing) versus field data.
This is the answer to why you’re failing Core Web Vitals or getting a Core Web Vitals Assessment Failed in PageSpeed Insights even with a high PageSpeed Insights Score
Table of Contents
What is “Lab Data” or “Synthetic Testing”
Lab Data/Synthetic Testing is what a speed test will report back. GTMetrix, SiteSpeedBot, Pagespeed Insights, Lighthouse, and every other test tool is doing a synthetic test. It’s emulating a real user and giving you the speed results based on its testing methodology.
It’s mimicking the real world but it’s not necessarily the real speed users are getting.
“What is “Field Data or “Real World Data” or “RUM”
Field Data/Real World Data/RUM is the actual speed in the real world as the user experiences it. CRUX data is field data, and data from RUM (Real User Monitoring) tools like Vital Signs Tracker is real world data, i.e. it records the actual speed of the web page when the user visits the page.
What we care about is speed in the real world because that’s what matters. This is why you can have a site that scores 100 in Pagespeed Insights yet still feels slow. Likewise, a lightning-fast site can potentially have a low speed test score.
Core Web Vitals: Assessment Failed!
You’re getting this error or notice in Google Pagespeed Insights because the real world speed of your site is slow.
It’s completely possible to have a 100 score in Pagespeed Insights and be completely failing CWV metrics.
When looking at these metrics in Pagespeed Insights, pay attention to which one is failing.
If you want to do a deep dive, use the CRUX Visualization Tool from Google.
A 100 Score in PageSpeed Insights is NOT the goal – beware of false guarantees
With our site speed optimization work, a 100 score in Google Pagespeed Insights is NOT the goal. Anyone who is guaranteeing you a 100 score is cheating the test in this tool!
Our goal is passing the Core Web Vitals metrics!
Essentially any tool or service that guarantees a 100 score is only partly loading the site so the PSI test only sees a partial website. This is most often done by delaying or not running the javascript on the site until the user interacts with it.
This looks great in a speed test (lab data) but in the real world can actually slow the site down! This is because it’s delaying the javascript and then when the user does go to interact with the site the javascript then loads adding more delay for the user.
Tools like Nitropack do this to cheat scores. On very light sites this is OK especially where there’s very little reliance on javascript. On a heavier site like a Woocommerce site, not only will this approach make things slower in the real world, it’ll often introduce CLS (cummulative layout shift) and break things that rely on javascript like forms and checkouts.
NOTE – delaying or pausing javascript in some circumstances also cause HUGE SEO problems because Google and other search crawlers only see a partial version of the site too. If you have a javascript heavy site, this can sometimes mean these crawlers are essentially seeing blank pages.
How To Monitor “Field Data” aka Core Web Vitals
Google’s Core Web Vitals metrics are fantastic. They give us a set of standardized metrics by which to measure the real world speed of a website. One problem CWV has though is that it’s based on a 28 day average. Depending on where you’re looking the dataset may be based on a 28 day rolling average or a monthly snapshot of the data.
Here’s some places where you can get field data or Core Web Vitals Reports:
CRUX Dashboard reports – a fantastic Looker Studio report that updates on the second Tuesday of each new month with the previous month’s data. This report is particularly good for agencies because it can be setup to email automatically.
The weakness of this tool is that it’s sitewide data. That makes it great for high level management discussions but not so useful for specific insights outside of which metrics are failing or are a problem.
CruxVis Tool from Google – this is an excellent tool for doing a technical deep dive into speed and Core Web Vitals metrics.
This is excellent for looking at historical data and tracking trends but again it’s high level sitewide data. It’s also very technical and a better tool for developers vs management or marketers.
Google Search Console – Google’s free SEO console showing you all sorts of data about crawling, indexing and Core Web Vitals
One real weakness of the Core Web Vitals data in GSC is that it’s based on groups of pages, not individual pages. It’s more precise at a page level that the previous two tools but still generally not precise enough.
RUM tools like Vital Signs Tracker – a problem we have with Google’s dataset is that it’s based on a 28 day average or a monthly data set which is why we built Vital Signs Tracker.
VST tracks every user session and can provide very detailed insights on site speed. It’s weakness is that it relies on javascript to pull the speed data so devices that are blocking javascript won’t be detected. It’s also emulating what Google is doing so it’s not a 100% exact match for Core Web Vitals data but is very close.
Click to enlarge on the image below to see an example report – this site is heavily failing Core Web Vitals!
Key Takeaways and Action Steps
At a minimum, start monitoring your “Field Data” or Core Web Vitals data on a monthly basis using the CRUX Dashboard report. This can be easily customized and tweaked to suit your specific requirements but gives you an excellent high level view of how your site is performing.
Support it with a RUM tool like Vital Signs Tracker which can then help you dig into the specific issues areas that are slow. VST also has an email reporting tool that can be customized in a variety of ways to email you reports on any schedule you like.
If you’d like help doing this in your business, request a FREE Audit on our homepage with some details as to what you’re looking for help with or where you’re stuck and one of the team will come back to you usually within 1 business day.