I think you will find that most WordPress sites display the same performance discrepancy between Mobile and Desktop.
For example, run these fairly famous WordPress-driven sites through PageSpeed Insights and see how they compare…
→ https://chicago.suntimes.com
→ https://www.katyperry.com
→ http://www.gsd.harvard.edu/
→ https://www.greenwichlibrary.org
→ https://www.obama.org
See what I mean?
PageSpeed Insights provides suggestions that you can implement to try to increases your speed scores.
This is not really a theme issue. For example, if you temporarily switch to a different theme and then run your site through PageSpeed Insights again I think you’ll find a similar discrepancy between the Mobile and Desktop speed scores.