[acqq]

By having most frequently used data grouped on, as you note, "small" stack instead spread all over heap you do have much better cache use. Moreover, even if the allocation is fast, if there are a lot of them, they just add up up vs. the case where there is nothing to be done when everything precalculated during the compiling phase.

If you haven't witnessed these effects I suspect it's only because you haven't had the reference to compare with.