TL;DR
If you read nothing more of this section, read this next paragraph and take it to heart:
Do not memorize rules in relation to CSS performance without checking your own data. They are largely useless, transient and too subjective. Instead become acquainted with tools and use them to reveal relevant data for your own scenario. This is basically the mantra the Chrome Developer relations folks have been promoting for years, I believe it was Paul Lewis (more of which below) that coined the term, Tools, not rules in relation to troubleshooting web performance.
Nowadays I get that sentiment. Really get it.