How much should we reduce the weight of selectors in css?

Question

css

Hi all!
What is the best (optimal) way to build selectors in css?
What is the fastest way for the browser?
is it worth replacing all selectors (nesting from 2 to 3 levels) with classes, for example, as if you throw it according to BEM, in the same place, in theory, only classes are used, nothing else.
From the point of view of readability, it seems pure classes are more convenient, but is it really necessary?