The main difference in the Google PageSpeed results between desktop and mobile is due to the way they're tested.
How is the mobile PageSpeed score tested by Google?
The mobile test is run with internet connection limits and a relatively-slow CPU. Currently, Google PageSpeed loads the web page as if it's on a mid-tier device (Moto G4) on a mobile network. The mobile network has these limits:
Latency: 150ms
Throughput: 1.6Mbps down / 750 Kbps up
Packet loss: none
More information about the network throttling can be found here.
Why is the mobile PageSpeed score low?
With this being the case, if your website is JavaScript-heavy, then you'll find that the less powerful mobile device - when compared to a desktop - will parse and process JavaScript much slower. This will lead to a longer load time. So it's important when optimising your site for mobile that you make sure that there is no unused JavaScript, and that any being used is well optimised.
How to improve a low PageSpeed score?
You can optimise the site with our Website Acceleration Suite, part of the free SIRSTEVEHQ CDN. You can use the JavaScript section to minify and optimised the JavaScript for the site and the image optimisation section to automatically create appropriately sized images for the mobile devices loading the site. More details on that here.
Unfortunately, this won't always get you a perfect score on mobile. This is because our tools won't ever remove Javascript from your site and Google PageSpeed Insights will still give a lower score for any unused Javascript on the page.
In most cases, to get a better score you'll need to make changes to the website's code, making sure that all JavaScript being loaded on any page is being used by that page.
