Hi @akahuyhoang ,
Things to try to resolve the problem:
a). [https://www.hardreset.info/devices/samsung/samsung-galaxy-a71/wipe-cache/|Wipe the cache partition] and check.
b). Start the phone in [https://www.hardreset.info/devices/samsung/samsung-galaxy-a71/safe-mode/|safe mode] and check if this improves the performance of the phone.
If it does then a ''downloaded'' app is the cause of the problem. The trick is to find out which one it is. You will have to uninstall each ''downloaded'' app, one by one and test the phone in normal mode between each uninstall. When it works OK then the last ''downloaded'' app that was uninstalled was the culprit. A ''downloaded'' app is one that you installed on the phone and did not come pre-installed on the phone when you got it.
c). Try a [https://www.hard-reset.org/factory-default/how-to-easily-master-format-samsung-galaxy-a71-with-safety-hard-reset/|factory data reset] (aka hard reset) and check.
''Be aware that a hard reset will erase all your data and downloaded apps. It will restore the phone to its factory default (or last system updated ) condition. If you decide to try this'' '''back up''' ''the phone to the cloud or a computer'' '''before''' ''performing the'' '''reset'''. ''Also if the phone has a Google account remember what the User name and password is for the account as you will need it to access the phone once the reset has been completed. (Google FRP lock)''
Once the backup has been completed, make sure that the battery is fully charged and then perform the reset.
Once the reset has been completed (be patient it takes a little while to complete) check if the phone now works OK i.e. no lag etc. If it does then using the backup that was made earlier, restore the phone using the phone’s restore feature (check in Settings).
Once the phone has been restored to what it was, check that it still works OK. If it does then the reset has resolved the problem.