PageSpeed Insights Update Thread

Lately we have been working to make desktop measurement more accurate.  Desktop scoring has been recalibrated to stay on par with the fast moving world of desktop browsing based on the most current data on global page speeds. You should expect desktop scores to be lower now than they had been. Mobile performance scoring should be unchanged.   

In addition to these changes to desktop measurement we have made some changes that improve user experience in more subtle ways:

• The mobile viewport scaling more accurately reflects the true screen size of a Nexus 5X.
• Audits that encounter warnings will now show a helpful warning inline.
• Lighthouse errors and the accessibility category are now localized to 42 languages.
• There's a new audit: FCP on 3G networks. It does not have an effect on performance scoring, but is useful for 3G diagnostics.

The headless Chrome infrastructure used by PageSpeed Insights had a bug that reported uncompressed (post-gzip) sizes as if they were the compressed transfer sizes. This led to incorrect calculations of the performance metrics and ultimately a lower score. The mailing list thread titled [BUG] [compression] Avoid enormous network payloads / Defer unused CSS doesn't consider compression covered this issue in greater detail. Thanks for Raul and David for their help.

The TL;DR: 

• Introducing Stack packs
• New report UI
• Lower global latency
• PWA now works 100%
• SEO and A11y available in 42 languages
• CrUX data and scoring updated

See the Lighthouse v5 on PSI Release notes for details and screenshots.

We do expect some changes to performance scores, though nothing too significant. 

They are detailed in the Changes that affect scoring  section.