At the code-writing stage, your emphasis should not be on optimizing: it should be entirely on functionality and producing correct bug-free code. Apart from optimizations (such as canonicalizing objects) that are good design, you should normally ignore performance while writing code. Performance tuning should be done after the code is functionally correct. Alan Knight wrote:
If testing and documentation are inadequate, most people won't notice or care how fast a particular list box updates. They'll have given up on the program before they ever got to that window.
 Smalltalk Report, March-April 1996. This is a nice article about when and why to performance-tune.
This is definitely a view to which I subscribe. Many implementation-level optimizations can make code more complicated and difficult to read. Delay optimizing until the program is mostly functionally correct. But make sure you have planned for a tuning phase.
I am not saying that you should create the whole application without considering performance until just before deployment. That would be foolhardy. Performance should be considered and planned for at all phases of the development process (especially design and architecture). You need to rule out designs that lead to a badly performing application. Optimizations that are good design should be applied as early as possible. When parts of the application are complete, they should be tuned. And benchmarks should be run as soon as possible: they give a good idea of where you are and how much effort will be needed for the tuning phase after code writing is mostly complete.