The interface of your app needs to render quickly, and interaction, such as scrolling through a list, for example, should run smoothly. In particular, older or low-end devices often have a hard time to do these things right. An unresponsive or slow UI can be the result, which is often caused by something that is called overdraw.
Overdraw is the phenomenon of a pixel on a view being drawn more than once. A colored background with a view on top of that has another background color is an example of overdraw (the pixel is drawn twice), but that's not really an issue. Too much overdraw, however, will have an impact on your app's performance.
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