When optimizing code, you probably want to work on performance improvements that can actually be fixed by editing the code.
Most of the things you can directly affect are things that happen in every test run, so best-case will include them.
Slower test runs will include events that don't happen on every test run (the computer is busy doing something else), so editing the code has less effect on them, and possibly none at all if it's completely unrelated.
Maybe those other events causing slowdown should be investigated too? But usually you want to look for a way to make them happen every time before working on them.
Most of the things you can directly affect are things that happen in every test run, so best-case will include them.
Slower test runs will include events that don't happen on every test run (the computer is busy doing something else), so editing the code has less effect on them, and possibly none at all if it's completely unrelated.
Maybe those other events causing slowdown should be investigated too? But usually you want to look for a way to make them happen every time before working on them.