Recently WhatsApp users have started seeing a message during chats (mostly noticeable during group chats) which reads
Waiting for this message. This may take a while
Why “Waiting for this message.This may take a while” appears in WhatsApp
If you are member of some very active WhatsApp groups and you turn on your Mobile Data / WiFi after few hour gap you would be flooded with group messages but messages with very little time difference would lose its integrity and it would not appear as how messages are sent, depending on message size (long text vs short text) .
To rectify the issue WhatsApp launched the above message “Waiting for this message. This may take a while” as a placeholder, so that the integrity of messages is kept and you get the Group messages in the way they are sent.