PageSpeed scores for your mobile site vary primarily because they use a different environment to test the page speed of your site. Mobile scores are tested using simulated mobile devices. Mobile computers typically have slower processors. Moreover, for mobile sites, PageSpeed Insights uses a cellphone network (2G, 3G, 4G) to test. Also, there are device-specific CSS rules for mobile devices that you must adhere to. These factors together cause the score for your mobile site to be lower than that of the desktop site.
Here are some tips that would be useful to improve your mobile speed:
Use compressed images.
Keep your website design simple.
Avoid or limit the use of custom fonts.
Reduce the number of redirects on the site as much as you can.
Minify the Code - Minification means simplifying the code and getting rid of any unnecessary characters (like empty spaces) without changing the functionalities. When you delete the excess code, you reduce the page weight which improve the speed and performance of your site.
I hope that these suggestions were helpful to you! If you need further assistance, I would be happy to analyze your site and make specific recommendations. So, if you’d like me to take a look at it then you can share the URL.