OK, regarding Sut Apps or similar...one more time...
Any software that attempts manage processes by closing down apps is counterproductive to Android operation. Again - Don't use any task killers, RAM memory optimizers/boosters, battery savers/repairers, cache cleaners, etc. They are counterproductive, waste power, disrupt system processes and degrade performance.
Let me try to explain a little more why this is so.
Manually futzing around closing apps, killing apps, installing RAM boosters, etc is simply incorrect operation of Android. Among other problems these apps cause, it means you're not taking advantage of Android's optimization capabilities, resulting in less than optimal performance and reduced battery life.
Android can learn from your usage habits and optimize itself to suit. It also learns when lag occurs or other system glitches and adjusts to cope with them better. Android actually runs better and better as you use it.
But it has to be allowed the chance to do so. If an app or the user intervenes and kills processes, the learning/optimization process is broken. The intervening app essentially causes incorrect usage information to be sent to Android, ultimately degrading performance.
The developers of Sut Apps, Advanced task Killer and many, many other apps clearly show a fundamental lack of knowledge of, or disregard for, basic principles of Android operation. They write apps to sell, not to actually be helpful, realizing that from a sales perspective, what sounds good is usually more profitable than what works well.
This link should help you understand. It's a good explanation of how Android handles multitasking, processes, RAM, etc automatically for written by one of the authors of Android:
Android Developers Blog: Multitasking the Android Way
Regarding Avast or any other antivirus...I've used Android since v1.0, from the beginning. I've never used or felt the need for any antivirus. To me they're just a waste of time and resources on your device.
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