Software development is the act of converting thoughts into actions. In other words goal of software development is to convert requirements into code but this can’t be done in one step and in a single iteration.
Each iteration in the software process you need to get a piece of requirement convert it into design, write a test, add new code and potentially refactor existing code to implement it.
For every piece in each iteration you want to follow the ordered sequence of correctness, completeness and performance. In other words, get it right, get it working, get it fast.
When understanding the requirement you want to make sure you are getting the right requirement basically understand what customers want not what they think they want. Often in requirement gathering phase the customer is biased to what they are used to or how they want something rather than letting you know the input and output they desire from a feature. Your job is to get to the bottom of it by asking good questions.
In the context of requirements completeness means that you not only understand the input and output but the entire context to implement the feature correctly. Once you understand what and in what context only then you turn your attention to performance requirements (if any) as discussion of performance earlier might not be in context and well understood where the customer is coming from. All these discussion do not have to happen at the same time but in should happen in that order.
I think I have already explained the hardest part in the whole iteration. Requirements is often where we get it wrong and the hardest/most-expensive one to correct after the fact.
When it comes to writing code you also follow this sequence, make sure you are implementing what you intend on implementing by writing unit tests first. The act of trying to write test first often force you to think about what you are about to do. You repeat this process until you have met all the requirements for the feature (completeness) and only then you measure performance and do any performance tuning if necessary. Software developers are not only often guilty of the thought crime of premature optimization but actually commit this crime if not stopped in time.
Do you think my thoughts are incorrect, incomplete and would not scale? Please send me a thought patch in the comments.