I had this issue on my XZ running Andriod version 7.1.1 I also realized that I had nearly 3GB of cached data on my device. Since clearing my Cached data the amount of RAM used by Andriod OS has dropped from 1.7GB to 1.3GB and my phone has become much more usable again. This Cache built up over about 6 months so this is my fix once every 6 month Delete cache then reboot. Would be good if Sony/Google (Not sure if this is a specific to just Sony Android or Android as a whole) had a way of cleaning this cache automatically as 3GB of Cached data for a phone is ridiculous. Clear cache instructions: Settings >> Storge & Memory >> Advanced >> Storage >> Internal Shared Storage >> Cached data >> Ok Hope this helps!
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