The performance of any application is vital for its success in the market. A user judges the quality of an application from its performance, whether it is the responsiveness, refreshing of frames (framerate), or memory usage (though indirectly perceived) of the app. All are important entities while determining any app's performance. Performance is one of the major factors for the application to compete with other apps of the same genres, and therefore, It needs to be critically tested and maximized.
There are numerous reasons for any app to exhibit a poor performance, some of which are summarized as follows:
Non responsive design while using it on multiple platforms and devices
The frame rate is not around 60Hz, which is best perceived by human eye
Synchronous AJAX calls, mainly in web apps
Loading of unnecessary CSS and JS files that are not needed in the first place for a page
Extensive use of memory consuming things, such as WebGL, 2D and 3D canvas, and so on...