Drastically different Google PageSpeed Insights “Lab Data” speeds between Mobile and Desktop experiences?

Question

When running the pages of this website through Google Pagespeed Insights tool, I receive drastically different "Lab Data" (Time to Interactive, First Contentful Paint, Speed Index) speeds when comparing Mobile and Desktop. Desktop tends to receive values under 2 seconds, and as a result, the Pagespeed Insights score is generally in the 80s or 90s on each page. The Mobile score, however, suggests the page load speed is much slower, upwards to 10 seconds. As you may guess, I cannot reproduce anything close to these loading times on mobile. The mobile and desktop experience do not differ dramatically with the primary differences being styling using CSS media queries. Would love any help understanding why these values are so dramatically different!

Images for reference: Desktop metrics Mobile metrics

Answer 1

Page Speed Insights uses simulated CPU and Connection throttling to simulate mobile conditions people may experience when displaying your mobile score (no throttling exists on Desktop score).

Not everyone has a flagship phone (far from it) so they slow the CPU speed of their server by a factor of 4 to simulate the slower CPU speeds of mid and low end phones.

Similarly they also simulate a slow 4G connection to account for when people are out and about / have no WiFi connection. SO they add additional latency and slow the upload and download speeds to reflect this.

This is why you see such big differences on your site score between mobile and desktop.

If you want to simulate a similar speed yourself you can open developer tools in Google Chrome -> Network -> Look for the drop down that says "online" and change it to "Fast 3G".

Now reload your page and you can see the effects of additional latency and slower download speeds on your waterfall.

Answer 2

According to my analysis, this is due to the images on this page. However, Google PageSpeed ​​Insights is very sensitive to mobile scores than desktop scores, so the stark difference between mobile and desktop scores is natural for this tool.

Try compressing the image first (you can use tinypng.com or other online tools), then use lazyload for image.