You might want to try some things.
First, make sure you’ve got at least 1GB free space on the device. iOS starts thrashing when free space is low.
Second, tap Settings > Battery to see if any apps are excessively burning the battery (and by extension, the CPU)
Third: Settings > Privacy > Location Services and make sure every app is set to Never, unless giving that app location services has some benefit to you.
Fourth: Settings > Background Refresh. Turn off everything except apps you want to refresh. For example, apps that sync data via iCloud.
Last suggestion: back up via iTunes, restore to factory settings, restore from backup. In my experience, this is iOS’ dirty little secret, sometimes it gets into a state that can only be resolved with a wipe.