Hi Tomas,
Thanks for reaching out.
With your selected optimization analysis tool, it’s best to follow it. Example, use a plugin that offers minification lie Autoptimize, W3 Total Cache, and even hosting specific optimization plugin like Pagespeed, or SG Cache, and so on.
Please note that it should be done on production mode of your site (a finished and live site). I don’t recommend doing that on your test site, which in summary, yes, you can’t achieve that high score while you’re on test and staging mode. Cache and optimization are the opposite of change, and what you’re doing on your test and staging are changes.
And to make it more faster, the optimization should make it static (like cache) and permanent so it doesn’t need reprocessing and re downloading, and it improves speed. That’s why it affects changes and updates.
And you must manage your caches and optimization for every change you made even if you’re already on production/live mode, especially the minification part. Cause it always has a bad result every time you change or update to your site. I recommend checking the documentation of the optimization and cache plugin or hosting feature.
Thanks!