Bad.  Optimizing before you know what to optimize.. usually by profiling
the application.  

Knuth called it the root of all evil.  

Is this homework?

Not good...the keyword being "premature", meaning "untimely, too soon,
hasty" in this context.

Premature optimization often refers to steps taken to improve speed or
reduce memory consumption, *before* there is reason to believe that such
steps are necessary. It does _not_ refer to judicious choice of fast and/or
memory-consumption-reducing architecture, design, algorithms and data
structures at the earliest possible stage.

AHS

I would like an example use of the word "premature"
where it has a positive connotation; I can't think of one.

 BugBear

On Tue, 29 Apr 2008 22:00:32 -0700 (PDT), tenxian
<hi.steven.tu@gmail.com> wrote, quoted or indirectly quoted someone
who said :

Stop jerking us around and ask real questions. Get yourself a project
and write some code.

(Typically it's considered bad form to put the main question as the
subject.)

Anyways, "premature optimization" (most likely deriving from the old
quote, "Premature optimization is the root of all evil," attributed to
Donald Knuth, author of TAOCP) is a simple phrase that is not idiomatic,
and thus can be parsed directly.

Premature - something happening before it should
Optimization - the process (or at least an attempt) of increasing
efficiency (with general connotation to time and somewhat so less so to
space)

Therefore, "premature optimization" is the process whereby the
efficiency is increased before it should be, or, cleaning up a bit,
"optimizing" code before one can say whether or not it should be. It
generally refers to people optimizing code based on where he or she
believes the code to be underperforming, instead of based on rigorous
tests which pinpoint the exact blocks which are consuming the most time.

As to your latter question, it is generally considered poor form.
Knuth's quote I've cited above, but another programmer (his name escapes
me at the moment) said something to the effect that "more sins are
committed in the name of optimization than any other cause--including
stupidity." I suspect it is up to decide to yourself, though...

OK, now I'm sure you're a troll.  Begone!