> Many recent CPUs eliminate NOPs without executing them

The elimination will still take some amount of time, and the smaller this amount is, the better, because it allows dialing in the slowdown more precisely. Of course how many NOPs you need for a particular basic block needs to be measured on the CPU you're profiling on, but that's also the case if you use a more consistently slow instruction, since the other instructions won't necessarily maintain a consistent speed relative to it.