Performance

Many things are important to consider when developing software. For example maintainability, robustness, quality, convention et cetera. One thing I have discovered as I have been developing software for many years is that performance is really critical. Slow and inefficient software has such a negative effect on user experience. This is especially true in the web and mobile world. You want to attract many, many users to your site or application, but if it starts to perform poorly with a large volume of users you have lost one of your main goals.

So next time you start hacking out a for loop, consider how many objects/records you could possibly be iterating over... It might be many more than you expect!