I guess most of the professionals working on the software industry have already heard the following quote about optimization.
Premature optimization is the root of all evil.
This quote was popularized by Donald E. Knuth on his paper Structured Programming with go to statements, what most of us ignore is that the quote, in his most acknowledged form was taken out of the context. A more complete quote goes like this:
We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.
Yet we should not pass up our opportunities in that critical 3%.
A good programmer will not be lulled into complacency by such reasoning, he will be wise to look carefully at the critical code.
As you can see, in a more complete context, the quote has quite a different perspective from ignoring the opportunity of optimizing your code prematurely.
You can, and you should, identify the 3% of your application that is really the critical path for your users, and the optimization of this part of the code should never be ignored, as it can cause you big problems at some point in the future.
Application performance is a subject that has been ignored for a few years with the increase of computational power, but now we are in a moment where users and systems demand the best possible performance from your application, so they can also deliver their results.
It is, of course, you option to identify this 3% as soon as you can, or to let your users find it for you. :)
Sources: Surviving Success