No dead CPU time in well-designed software. If you care about I/O performance then you are managing that in user space asynchronously. No thread is waiting for I/O to complete or anything like that unless there is literally nothing else to do (in which case more threads won't help).
There is no performance benefit to adding more threads, they would increase CPU state/cache thrashing which will slow things down. The main benefit of using a lot of threads is that more programmers know how to design software that way because it was taught for so many years; it isn't good for performance on modern hardware.
There is no performance benefit to adding more threads, they would increase CPU state/cache thrashing which will slow things down. The main benefit of using a lot of threads is that more programmers know how to design software that way because it was taught for so many years; it isn't good for performance on modern hardware.