Qt Quick applications are supposed to be fast, with smooth and fluid user interfaces. In many cases, this is easy to accomplish with QML; the contributors to QML and the Qt Quick runtime have put a great deal of effort into creating an environment that performs well under a wide variety of circumstances. Sometimes, however, try as you might, you might find that you just can't squeeze the performance out of your application that you'd like. Some mistakes are obvious, such as the following:
Doing a lot of compute-intensive tasks between state changes or actions that trigger drawing operations
Excessively complex view hierarchies with thousands of elements on display at once
Running on very limited hardware (often in combination with the first two problems)
Knuth famously said, "Premature optimization is the root of all evil," and he's definitely right. However, there might come a time when you will need to measure the performance of your application, and Qt Creator...