Why is there a difference between GSC and Semrush Core Web Vitals data?

You may notice a difference in Core Web Vitals values reported in Google Search Console (GSC) and our Site Audit report. The difference exists because these two reports have different data collection methods.

GSC Core Web Vitals data comes from the CrUX report. This report collects anonymized field data based on actual users. 

The primary tool that we use in our Site Audit for Core Web Vitals data collection is Google Lighthouse. Google Lighthouse is an automated, open-source program that can measure Core Web Vitals in a lab environment with a simulated load of a page.

Field data is affected by different devices and network conditions, while lab data is collected in a fixed environment. This is why it is not unusual to see the difference in values.

The Lab environment is best for assessing your optimization efforts when you are in a process of improving your website performance (since you do not need to wait for the gathering of field data). And if Site Audit metrics are improving, GSC Core Web Vitals metrics should improve as well.