Why does my Galaxy 4 take forever to send/receive texts after getting/sending an MMS?

[AC Question]

Whenever I send or receive a photo, group message or anything else classified as an MMS, for 5-10 mins afterward it takes forever for me to send or receive a text message (SMS). I can send one or 10, to the same person or to other people, and the SMS just sits there with the "sending" circle animation underneath it. People always think I'm ignoring them when we're engaged in conversation via text, then they send a photo, and it takes forever for them to get my response!