Why is my Google PageSpeed score lower on mobile?
The main difference in the Google PageSpeed results between desktop and mobile is due to the way they're tested.
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:
Throughput: 1.6Mbps down / 750 Kbps up
Packet loss: none
More information about the network throttling can be found here.