When an Android phone starts to feel sluggish, a factory reset is often the first solution people think of. The thinking is that each new Android version adds features, more demanding animations, and extra security protections that gradually make older phones feel slower. Android often spends the next day or two rebuilding app caches, re-indexing files, and performing other post-update optimization and cleanup tasks. There are the usual bug fixes and compatibility improvements, but you might even see some performance optimizations that make your phone feel faster, not slower. One way to identify these apps is to dive into your phone's Battery settings and check Battery Usage.