not just read-time, but yes it is that as well, I find a class I search for this class, now I need to understand all scope in html tree and scope of the css to understand am I looking at the right one (sure you can run into situations with that as well, but in my experience nesting makes it more difficult)

but it is also a source of bugs, because complicated selectors mess with css priority and the cascade then someone writes something that has side effects and blame CSS instead of their ridiculously long nesting.