Replying to text messages is considered very important to people because this is a form/Care language thus, people who don't reply fast enough or leave them on seen, and delivered for long periods of time it makes the other person feel left out, uncared for, and unloved. These tend to make people overthink their relationships whether it is friendships, or romantic relationships, However, with the same token, it may show the party who left the person on seen or delivered may be feeling sad or mad. In most cases thankfully, this tends to be the reason for the party to leave one on seen or delivered (Mostly in Romantic Relations).
Now, if I were to text you a simple, "WYD" you should not respond with a "hey" the correct response is, "Hiiiiiiiiiiiiiiiiiiiiiii" with a bunch of I's or "Heyyyyyyyyyyy" with a bunch of Y's.
In conclusion, when my text reaches you, you better hit me with an eager excited immediate response.