“My site feels slow” is not something you can fix. A number is. Performance metrics turn a vague feeling into something you can measure, improve and track. Here are the ones that matter, what they mean and what a good result looks like.
Metrics about loading
Time to First Byte (TTFB)
How long the server takes to start responding. Google’s guidance treats 0.8 seconds or less as good. It is the foundation for every other loading metric. Learn more in our guide to TTFB.
First Contentful Paint (FCP)
When the first piece of content, such as text or an image, appears on screen. Good is 1.8 seconds or less. It is the moment a visitor knows something is happening.
Largest Contentful Paint (LCP)
When the largest visible element finishes rendering, usually a hero image or main heading. Good is 2.5 seconds or less. LCP is one of the three Core Web Vitals.
Speed Index
How quickly the visible part of the page fills in during loading. It is a lab metric from Lighthouse, and lower is better.
Metrics about interactivity
Interaction to Next Paint (INP)
How quickly the page responds when someone clicks, taps or types. Good is 200 milliseconds or less. INP is a Core Web Vital and replaced First Input Delay in March 2024.
Total Blocking Time (TBT)
The total time during loading when the main thread was blocked long enough to make the page feel unresponsive. It is a lab metric and is a useful stand-in for INP while you are testing.
Metrics about visual stability
Cumulative Layout Shift (CLS)
How much content moves unexpectedly while the page loads. Good is 0.1 or less. Set image dimensions and reserve space for banners and embeds to keep it low.
Page weight and requests
Total page size and the number of requests are not Core Web Vitals, but they influence everything above. Smaller pages and fewer files load faster, especially on mobile networks.
Lab data versus field data
- Lab data comes from a simulated test, such as Lighthouse. It is repeatable, and perfect for finding and fixing problems.
- Field data comes from real visitors, such as the Chrome User Experience Report and Search Console. It is what Google uses for page experience, and it reflects real devices and networks.
Use both. Lab tests tell you what to fix. Field data tells you whether it worked.
Tools to measure with
- PageSpeed Insights shows lab and field data for a URL.
- Lighthouse in Chrome DevTools runs detailed audits.
- WebPageTest gives a full waterfall and the timing of each step.
- Search Console has a Core Web Vitals report for your whole site.
How to read your results
- Start with Core Web Vitals, because those affect search.
- Look at the opportunities and diagnostics for the specific causes.
- Fix the biggest items first, and re-test after each change.
- Test on mobile, since that is where most sites score lowest.
- Run several tests, because results vary from run to run.
Set a performance budget
A budget is a limit you agree to stay within, such as an LCP under 2.5 seconds or a home page under 1.5 MB. Budgets stop a site from slowly getting heavier as new features and plugins are added.
Where to go next
Our WordPress speed checklist turns these metrics into an ordered list of fixes, and our WordPress speed optimization service can do the work for you. Contact us for a free review.
