Last Thursday, January 16th, I joined TJ Gamble's livestream to answer some PageSpeed related questions. Critical CSS was one of the subjects. Just before the livestream, I published a code snippet to auto-enable CSS critical Path within Magento 1.
As of version 2.3.3, Magento introduced CSS critical path. Users will experience a faster loading product-page when applied (correctly)!
Have you been to Magento Live in Amsterdam and did you speak the Google PageSpeed crew about your pagespeed score? I already heard some e-commerce owners, the Google PageSpeed crew might have given you some wrong insights, which I am going to answer instead.
You want your e-commerce to use most supplier-efficient and cost friendly delivering process. At the same time, you want to keep calculations and thus memory outbursts to a minimum.