CPU&Memory usage Monitor for Android
Use it to understand which apps "eat" too big part of your device's cpu or memory. I hope it'll help you to find them and make your device faster.
Note, that it is absolutely free with no ads or in-app purchases.
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This app is published under GNU GPL v3.0 license. You can find its source code here: https://github.com/danpetruk/CPUMemoryUsageMonitor . Other information you can find also there.