Sometimes performance is not the main factor

Note, it is also design decisions that may not by the best for performance. Some of the issues is with the compiler. We break large functions up to make it more readable. This creates hard issues about inlining functions or not.

You may think inlining a bunch of functions will help in performance, but then you may increase the size of the code and start taking more instruction cache misses, which cost more than a function call. Some archs handle function calls better than others.

Yes, if a design improves the code by 1 or 2 percent, that may be rational to go with the more complex design. But if the more complex design only saves you a quarter a percent, and it is much more likely to carry bugs (more complex code is always more buggy) then it is not worth it. But as the kernel grows, each of those 1/4 percent performance regression adds up.

With things like ftrace and perf now in the kernel, we can start looking deeper at problem areas, and hopefully redesign things in a maintainable way to get some of our performance back.