What should girls do if they reply slowly by texting?

Never. You have to talk to her for a long time, find out what her personality is, and then consider the countermeasures. If a girl is shy, you can boldly encourage her to chat with you. If she doesn't like you, you may ask, and she may directly hack you and delete you. Either way, you can impress her with high emotional intelligence. You can say that I miss you and have been waiting for your reply. For the sake of my sincerity, can you call me back as soon as possible?