Yeah, interesting observation. I guess we can expand this further, leaving all non-techies way behind…
So - If you’re not completely using the full capacity of 1 thread, then engaging further threads without strictly needing to, is definately not free (see Ingo’s measurement above), and incurs quite the overhead.
But - As you point out, if you max. out 1 thread with the load of X, then the next thread you engage can also be loaded with X, and the next, and the next, which means that the “total patch capacity” of Rack seemingly scales very nicely with the thread count. Which is very good to know if you have a beefy computer with great cooling, and would like to run really big patches.
That, however, does not make the overhead on the total load on your computer’s CPU go away. If you have a monster computer that doesn’t matter at all, but with a more modest/normal computer, and especially a laptop, the overhead definately counts, because what matters is how your computer copes with the total load you put on it’s CPU, especially where cooling is concerned.
So I will still say: Single core performance matters a great deal, particularly on low/mid-range computers and especially on laptops, because the better a single core performs, the bigger the patch you can run on just 1 thread, before you have to engage the next thread, and pay the overhead cost of multithreading on your CPU.