If your shiny new handset is starting to feel sluggish under the weight of all of the apps, themes, and modifications you've made to it, then you might like to try out Diagnosis for Android. The app provides comprehensive system information on your phone's CPU, RAM, and battery use, along with other details like how fast your storage can be written to and read, as well as data on your network connections. It'll also show you the CPU and RAM use app-by-app, so you can see if a particular piece of software running in the background is eating up your resources (looking at you, eBay).
The basic version is free, though a Pro key unlocks additional options like autostart and notifications. It's quite addictive to see how your phone behaves as you throw different tasks at it — a routine task like updating apps from Google Play is surprisingly intensive on the processor — and being able to look into your phone's hardware in this detail could be handy if you're ever experiencing issues. If you want to give it a try, you can download the free version from the Play store.