I don't think it's useful to think like this when writing software. Sure you must do more work when going multi threaded. But that doesn't mean you are slower in wall clock time. And wall clock time is the cool kid.
I mean if you go this route then you may as well say zero-copy doesn't exist. Everytime you move things between registers things are copied. I guess OP also disables all their cores and runs their OS on a single core. It's more efficient after all. I take the other view. The more effective CPU time you can use the better for a ton of non UI use cases. So I would say it is more efficient to use 2x the CPU time to reduce wall clock time by 10% for example. In fact any CPU time that is unused is inefficient in a way. It's just sitting there unused, forever lost to time.
I mean if you go this route then you may as well say zero-copy doesn't exist. Everytime you move things between registers things are copied. I guess OP also disables all their cores and runs their OS on a single core. It's more efficient after all. I take the other view. The more effective CPU time you can use the better for a ton of non UI use cases. So I would say it is more efficient to use 2x the CPU time to reduce wall clock time by 10% for example. In fact any CPU time that is unused is inefficient in a way. It's just sitting there unused, forever lost to time.