You find the 'hottest' code (usually a loop, the code executed most) and examine very carefully the generated assembly for that and how it can be made more CPU/JIT/VM-friendly.

This is an optimisation you perform when a small amount of code is consuming the overwhelming majority of your software's execution time (code very commonly has this property, until it is profiled and optimised).

In addition, this case is for a high speed networking library, where every CPU cycle counts.