Modern CPUs can do 4-5 GHz singled threaded. (Sometimes you can even get a higher clock speed by disabling other cores.) This somewhat outpaces "a 1mhz 6502" even without parallelization.

They can, but nobody runs a single process on such CPUs. They run some form of OS which implements spinlock, mutexes, and all these other complex things.

I suppose someplace someone is running an embedded system without an OS on such a processor - but I'd expect they are still using extra cores and so have all of the above tricks someplace.