Do a backup and factory reset it after os update like major update it will be like this. I have used mi a1 stock Android it also had same issue after major update battery will be reduced and performance also. Once reset it should work.
Check your storage space on the device and delete unnecessary files. Create some free storage space to make your phone perform better. Remove files and apps that you don't need by using Device care. Check your storage space and select files and apps that you want to delete
