Due to low specs of the phone. This is a common problem in those phones that have low-end processors and ram. If u reset ur phone or delete some of the apps, u may not see that problem again
Because many apps start to work in background that require internet to refresh and send you notification or work in background. Here's one solution
From battery menu, Put all apps in deep sleep and your phone will work fine if not great. Just don't put system apps and communication apps. Apps work fine even in deep sleep mode. Deep sleep just removes their ability to function at background so those apps will only work when you open them. You can even put calculator in deep sleep because you don't need it to work in background, nor you'll be needing any of its notification. But Don't put calender or clock in deep sleep otherwise you'll miss their alarms and alerts. And if you don't have youtube premium, put youtube in deep sleep too. Premium allows youtube to run in background so if you have youtube premium then don't put it in deep sleep. Your phone will work fine
