Exactly how “fast” are modern CPUs?

Question

When I used to program embedded systems and early 8/16-bit PCs (6502, 68K, 8086) I had a pretty good handle on exacly how long (in nanoseconds or microseconds) each instruct

Answer 1

As Doug already noted, the best case is zero (superscalar processor, multiple execution units, data already in L1 cache).

The worst case is up to several miliseconds (when the OS handles a pagefault and has to fetch the data/instruction from the disk). Excluding disk/swapping it still depends on whether you have a NUMA machine, which kind of topology it has, in which memory node the data lies, whether there is concurrent access from another CPU (bus locking and cache synchronization protocols), etc.