Yii is a high-performance component-based framework for developing web applications on any scale. It encourages maximum code reuse in web programming, and can significantly accelerate the development process. As mentioned previously, if you stick with Yii's built-in conventions, you can get your application up and running with little to no manual configuration.
Yii is also designed to help you with DRY development. DRY (Don't Repeat Yourself) is a key concept of agile application development. All Yii applications are built using the Model-View-Controller (MVC) architecture. Yii enforces this development pattern by providing a place to keep each piece of your MVC code. This minimizes duplication and helps promote code reuse and ease of maintainability. The less code you need to write, the less time it takes to get your application to market. Similarly, the easier it is to maintain your application, the longer it will stay on the market.
Of course, the framework is not just efficient to use, it is also remarkably fast and performance is optimized. Yii has been developed with performance optimization in mind from the very beginning, and the result is that it is one of the most efficient PHP frameworks around. The Yii development team has performed performance comparison tests with many other PHP frameworks, and Yii outperformed them all. This means that the additional overhead Yii adds to applications written on top of it is negligible.