It is more important to ensure that your code is correct than fast. Fast performance is not of any use if the application does not yield the correct results. Having said that, performance is an important part of designing and developing a successful software application. It plays a large part in the overall user experience for people who use the application. Regardless of the device they are using or their location, users expect a high level of responsiveness from their applications.
Performance is a quality attribute of the software system and performance requirements should be documented. Like all requirements, they need to be measurable and testable. The entire team should take ownership of performance. A systematic, iterative approach to performance improvement can help a development team reach their performance goals. Some problems will only be discovered later, so...