Above the fold (atf) optimization is a compex issue. It should not be left to DNN.
For instance, how do you determine what is above the fold? My monitor is 3840 x 2160 but my iPhone 4 is a lot smaller.
If you use a program to determine atf, how does it click buttons, focus, touch, hover, change resolutions, etc.? Do you want your drop-down menus to work? There are some grunt and gulp tasks to do this but none have solved all problems. It still needs some human intervention.
If you inline your atf css and load your main style sheet via js, it is not cached. Now you need a cookie to check whether to serve from cache.
This atf tests better (on webpagetest.org) than it actually works.
Some of the larger sites that use this, the page bounces up and down like a cheap hooker and text reflows, etc. Do you want it fast or good?
Mobile - Over 25% of users scroll before your page can even load, and then spend 66% of their attention below the fold. (Why are you ignoring below the fold?)
As for your poor rating, in Google PageSpeed I can get 92% non-DNN and 86% (green) on a DNN website (I need to optimize this - should be able to hit 90%)
If your score is in the red, I suggest you read some of the DNN optimizing techniques mentioned in this forum.
I have never been able to get Client Resources Management feature to do minification and combination of js and CSS files to work on DNN, so use a gulp task instead. Just combining css is not the answer, you need to declutter, dedupe and remove useless style sheets.
Please, let's just get DNN to fix the bugs, add enhancements to DNN and leave this to SEO, CSS or other experts. DNN is a kitchen sink framework, not an optimizing tool.
Do you want to build your site for Google PageSpeed or humans?