> When that 5800X cranks up 8 cores, it dramatically slows down the clocks

Not that dramatic, it drops from ~4.8ghz to ~4.4ghz: https://www.anandtech.com/show/16214/amd-zen-3-ryzen-deep-di...

Actual drop varying depending on actual power consumption & temperature as Ryzen is more or less an entirely reactive system.