According to Kewin Charron, a senior lead refurbishment operations manager at BackMarket.com who specializes in refurbishing old handsets, the number one reason phones run slowly is because many apps run in the background, using up the phone’s RAM and CPU. Users are often unaware of this. Charron recommends visiting your phone’s settings menu to turn off battery-intensive apps and optimize your handset with a few settings tweaks.
Kewin Charron says there are a few obvious problems that could be slowing your handset
Charron warns that when a phone’s storage is nearly full, it has less memory to work with, which can also slow things down. To avoid this, delete old applications and videos or move them to cloud storage. For a more thorough cleansing, backup your phone, restore it to default settings and finally restore the backup version. Battery issues are also a common cause of slow phone performance. Charron recommends using settings and apps, like AccuBattery, to monitor battery health and to replace the battery if necessary.
Background apps can be disabled using certain settings on iPhone and Android