A "Which is faster?" question might be stupid in 99% of the cases seen from an economic point of view, but the threads here are typically the more interesting ones. I remember the discussion about using match variables.
And of course, in many cases, people ask here because CPU cycles matter, simply because their script is too slow.
Btw: it is simple (at least in theory) to calculate whether a particular optimization is economically a good thing. just run the profiler and apply http://en.wikipedia.org/wiki/Amdahl's_law.