Nowadays having a score of about 40/50 on mobile is a great result, reaching higher values is very difficult given the evaluation criteria of the PageSpeed test for mobile devices and would certainly require manual interventions with long and complex activities, therefore not simply based on an automated plugin such as JSpeed.
To reach a higher score most probably it would be necessary to refactor several aspects of the website such as:
- Convert and replace all site images using the webp format
- Remove all unnecessary or heavy extensions
- Manually edit the template to remove optional assets
- Manually edit scripts and css files to remove all unnecessary code
- Improve the server performance