Imagine a personal vehicle that is roomy, comfortable, fuel efficient, inexpensive to produce, and 98% recyclable. You want one? Sure. Everyone does. Just one problem. Its top speed is 6 miles/hour (10 km/hour). Performance does matter.
Whenever you design a product, whether it be a consumer product or a computer system, performance should always be a primary consideration. The product might be easy to use, inexpensive to produce, and yet unusable because of performance.
The reason many designers put performance at the bottom of their lists might be that computers are so much faster at computation than their human counterparts, and therefore they assume that the speed of the system will be acceptable. And if today's systems aren't fast enough, Moore's Law will take care of everything.
Performance is sometimes thought of as a simple measurement of the time it takes for a system to respond to user input. But system designers must consider many aspects of performance, including performance of the analysts and programmers who implement the design; performance of the human interactions of the system; and performance of the offline, or non-interactive, components.
Performance of the people building the system is often called productivity, and it is important because it directly affects the cost and schedule of the project. A project that is delivered late and over budget has a lot of 'splainin' to do. Using tools and pre-built components can dramatically affect how quickly the system can be built and start returning value.
Performance of the human interactions is critical to acceptance of the system. Many factors of system design contribute to this aspect of performance, response time being perhaps the most obvious. But response time is not the only factor. Just as important are intuitiveness of the interface, the number of gestures required to achieve a goal, and the amount of information presented to the user.
More than response time per se, a good system specification will measure task time, defined as the time required to complete a domain-specific task, including all human interactions with the system. In addition to system response time, this measurement includes operator think time and operator data entry time which are partly not under the control of the system. But including these times gives motivation to the proper design of the human interface. Proper attention to the way information is presented and the number of gestures required to complete the task will result in better human operational performance.
Performance of the offline components is equally important to the success of the system. For example, a "nightly" batch run that takes more than 24 hours to complete will result in an unusable system. Performance of the disaster recovery component is also a critical consideration. In case of total destruction of one part of the system, how quickly can operational status be restored, in order to allow normal business to resume?
When considering the implementation and operation of a successful system, architects and designers should always pay careful attention to performance.