>Meanwhile, it’s been 15 years since my first 2 GHz+ MacBook, and a new MacBook still has a base clock around that range. Maybe it’ll turbo boost to 4 GHz for 10 seconds before it exceeds its thermal envelope.)

The short version is that MacBook is not designed for speed/performance but looks. It overheats by default.

2700k could reach/overclock to 4.5Ghz w/o much trouble, given proper cooling - 2011. The reason for lower base clock is the Intel TDP definition.