Improve performance for PWA

How can I optimize the theme to improve PWA loading performance?
(audits done via Page Insight)

Currently, my only problem that I didn’t manage to fix is: ‘Remove unused CSS’. After checking the static folder, I saw the CSS quantity is consistent. Using minify plugins somewhat improved the load time.

I must also add that I use some 30 lines of Additional CSS to tweak the design where the customizer couldn’t. But that’s not in the Unused CSS category right?

My wordpress site will be used mainly as a PWA.