Normally, mobile devices use four primary pieces of hardware to render a web page on to your screen. The CPU calculates all the display units, the GPU renders all the images to the screen, the memory stores all the data, and the battery provides the necessary electric power. All of these hardware units have their own limitations. Forcing or exceeding those limitations will cause your application or website to become slow, have a rendering issue, or will drain the battery.
To discover the reasons for specific performance issues and to find a solution, you have to take a look at your application's backend layer, use a tool to collect data about your application's behavior, generate a report with graphics, study those reports, and enhance your code.
Nowadays, many mobile development platforms and mobile devices provide profiling tools to generate visualized reports such as rendering, computing, and the battery performance of your application.