[amelius]

And what if the cache line is full/dirty?

[bonzini]

The cache coherency protocol takes care of that. In other words the first part is just a memory load and can vary from 0 to a few hundred clock cycles, the second is local to the processor and has a more or less fixed cost. The worst-case execution time is completely dominated by the first part, the best case instead is dominated by the second.

[jasonwatkinspdx]

I'd suggest reading the wiki articles about it for an introduction, and Ch 5 of https://www.amazon.com/Computer-Architecture-Quantitative-Jo... for a detailed understanding.

Right now you're asserting things about all this, while not being familiar with relatively basic aspects of how it works.