WhatsApp for Samsung Galaxy Core Lite LTE

Samsung's aggressive battery saving is the usual cause: go to Settings > Apps > WhatsApp > Battery and set it to Unrestricted, then under Settings > Battery and device care > Battery, turn off Put unused apps to sleep and add WhatsApp to the Never sleeping apps list. Also confirm notifications are enabled in Settings > Apps > WhatsApp > Notifications, and that Do Not Disturb is off. See how others solved this on Reddit

WhatsApp drops support for old Android versions over time, so first check Settings > Software update and install whatever is available. If your Android version is too old for the current app, install WhatsApp from the Play Store while it still supports your build, and keep your chats backed up to Google Drive (WhatsApp > Settings > Chats > Chat backup) so you don't lose messages if the app finally becomes incompatible. See how others solved this on Reddit

On low-RAM Samsung phones, clear WhatsApp's cache via Settings > Apps > WhatsApp > Storage > Clear cache, and trim large media in WhatsApp under Settings > Storage and data > Manage storage. Turning off Put unused apps to sleep for WhatsApp (Settings > Battery and device care > Background usage limits) keeps it from being killed and reloaded, which is what causes the worst lag. See how others solved this on Reddit

Open WhatsApp > Settings > Storage and data > Manage storage to find the largest files and forwarded media, then delete what you don't need. To stop new media from piling up, turn off auto-download under Settings > Storage and data > Media auto-download (uncheck Photos, Audio, Video and Documents for mobile and Wi-Fi), and keep your backup on Google Drive instead of the device. See how others solved this on Reddit

On your old phone, open WhatsApp > Settings > Chats > Chat backup and run a backup to Google Drive using the same Google account you'll sign into on the Core Lite LTE. Then install WhatsApp on the new phone, verify the same phone number, and when prompted choose Restore so your chats and media are pulled back from Google Drive. See how others solved this on Reddit

Make sure WhatsApp has microphone and notification permissions under Settings > Apps > WhatsApp > Permissions, and set its battery usage to Unrestricted (Settings > Apps > WhatsApp > Battery) so incoming calls can wake the app. Also disable Put unused apps to sleep for WhatsApp and any task-killer behavior, and confirm you have a stable Wi-Fi or mobile data connection, since a weak LTE signal will cut voice and video calls. See how others solved this on Reddit