Comment by kragen

It's true that this code was optimized from 2.6ns down to 0.9ns, a saving of 1.7ns, while an L2 cache miss might be 80ns. But 1.7ns is still about 2% of the 80ns, and it's about 70% of the 2.6ns. You don't want to start optimizing by reducing things that are 2% of your cost, but 2% isn't insignificant.

The bigger issue is that probably you don't need to do leap-year checks very often so probably your leap-year check isn't the place to focus unless it's, like, sending a SQL query across a data center or something.