This is critical since many optimizations are accomplished by violating (hopefully safely) module boundaries; it is disastrous to incorporate optimizations into the main body of code. The separation allows the optimizations to be checked against the meanings.

It's typically taken for granted that better performance must require higher complexity. But I've often had the experience that making some component of a system faster allows the system as a whole to be simpler

The latest SQLite 3.8.7 alpha version is 50% faster than the 3.7.17 release from 16 months ago. [...] This is 50% faster at the low-level grunt work of moving bits on and off disk and search b-trees. We have achieved this by incorporating hundreds of micro-optimizations. Each micro-optimization might improve the performance by as little as 0.05%. If we get one that improves performance by 0.25%, that is considered a huge win. Each of these optimizations is unmeasurable on a real-world system (we have to use cachegrind to get repeatable run-times) but if you do enough of them, they add up.