Ah, see—this was an issue many people have had. I didn't have this issue, but sometimes using Smart Switch on your apps messes everything up on older versions. Make sure all apps are updated in the Samsung store and Google store; it's sometimes best to factory reset and move everything besides the apps.
Secondly, clear the cache, as older versions may be bugging the phone.
Plug the phone into a PC, then turn off the device—your device won't boot to Recovery Mode when it's on. Press and hold the Volume Up key and Power key. Once the Android Recovery screen appears, release the buttons. Use the Volume Down button until you are highlighting "clear cache partition." Press the Power button to select "Wipe cache partition." Use Volume Down to highlight Yes. Press the Power button to confirm the action to clear the cache. Wait a second as the system wipes the cache. Press the Power button again to reboot the device.
Go to System Care.
Do an auto clean.
Make sure you go to Apps and clear any unused apps.
See if there are any background apps running, too.
Ps: in terms of battery remember it takes 2 weeks to fully optimise. My battery was dead after a good 6 hours and now is double that at 12/14 hours.