I was pulling my hair out trying to figure out why I was seeing SMS delays no matter whether using the stock Messages program or other third-party SMS programs. After much time spent diagnosing this (including backing up, wiping, and reloading the phone), I finally figured out what was going on. I had not seen this solution anywhere on the internet. So I am offering it here in hopes that it will help others.
I had read about making sure that power management was turned off for the messaging program(s), and I made sure that it was. But I have McAfee installed as my virus checker on my Galaxy S8+, and I finally came to realize that it was being power-managed. Once I turned off power management of the McAfee program, all the delays went away!
Apparently, when the virus checker is hibernated, it is not awake to immediately respond to messages. Hence the delay. Making sure that your virus checker is not power-managed is the key to keeping it awake and responsive to messages!
That just started happening to my Samsung Galaxy s8+ on Sprint. I'm using "Android Messages" app from Google Play store. My sent messages were being delayed by about 2 minutes, so I went back to the stock messages app and that worked fine. Then I went back to my go to message app and that started working just fine after the back and forth switch. Hope this helps.
I have been having the same problems with delyaed text messages. I switched over from the Iphone thinking it might be a better phone for myself. I have had more problems with the galaxy s8 note than any of my iphones. I send a single text and my contancts get multiple of the same messages. I will send a message and it says not deleivered even though my contacts got it. I sent a picture the other day and my contact did not get it until over 12 hours even though it said it had sent. They were right next too me with full service. I don't know why I waste $1,000 on a phone that can not even handle simple text messages. Really frustrating.... And the service is also worse with the droid. I only changed my phone, not service, and now I have even worse service every where I go.