[LRUG] Writing specs for the time-complexity of a method

[Roland Swingler]

> to actually get a difference you could measure reliably you'd need to work
> with large sets of data and thus slow your test suite down.

Could you not separate out performance related specs from the main
test suite? You're likely to have far fewer performance specs, and
they probably wouldn't need to be run as part of the main
red-green-refactor cycle - in which case it matters less if one test
took a couple of minutes to run.