UPDATE regarding CPU usage for Android Oreo and up. So Android Oreo added yet another restriction disallowing access to the CPU usage stats. Thus, CPU usages unfortunately won't work on devices with Android Oreo. I have released a beta version of the app accommodating the rooted Android Oreo devices where it should work just fine.
It's free of charge to get the .apk file of Simple System Monitor and you can take a look at users' reviews on Droid Informer. This application installation package requires 3 MB of free space to run on Android 4.0 and higher. According to the antivirus report, the app is safe and secure. Darshan Parajuli is the official developer of this application which can be found in the Tools category.