Well it's entirely possible that Knuth and I disagree here, but if you architect an application without thinking about performance, you're likely going to make

Well it's entirely possible that Knuth and I disagree here, but if you architect an application without thinking about performance, you're likely going to make regrettable decisions that you won't be able to reverse. It is not possible to predict bottlenecks in computation, no. But the implications of putting global state behind a mutex in a concurrent application should be clear to the programmer, and they should think seriously before making a choice like that. If you think of a different way to do that while the code is still being written, you'll avoid trapping yourself in an irreversible decision.