I had the impression, the turbo button was created to slow down new PCs, so they could run old software that relied heavily on CPU speed.

It’s not worth optimizing for situations that do not occur in practice.

The transistors used to detect register clearing using XOR foo,foo, on the other hand, are worth it, as lots of code has that instruction, and removing the data dependency (the instruction technically uses the contents of the foo register, but its result is independent of its value) can speed up code a lot.