[yazaddaruvala]

Why? You're assuming there is no sharable code.

Instead of sharing the code at runtime, i.e. what an OS does. You could easily share code at compile time, i.e. statically link a library.

Because of sharable code, "implementing * in application" should always be at-least as performant as the best generic implementation (i.e. the implementation you find in a general purpose OS). However, when appropriate, customizing the implementation for the application would allow it to become even more performant.