So I was having this issue on my Note8 a few days after I bought it, which was last Saturday. I did the smart switch thing from my S8, and decided to wipe my phone and start fresh, installing apps occasionally and testing messages as I go.
I went about 4 days with no problems and sent and received a few messages with a colleague today, with no problems. Then, this evening at work, it started again. Messages would either "send" for several minutes (I screen captured several and had delays of up to 15 minutes), and a few just flat out failed, according to the phone.
I have Digits and installed the app. Sending and receiving messages to and from digits is almost instant. Even messages I send to my Digits number from the stock message app, appear in Digits within a couple of seconds, but the stock message app shows sending for quite some time. Replies are sent, from the Digits app, and received on my other number in Digits almost immediately again, but are delayed being received by the stock messaging app by several minutes, and usually won't show received until the initial message shows sent.
What this says to me is that the messages are being received by T-Mobile, but there is a problem somewhere on the software side of things, or in the connection between the messaging app and the messaging service.
A few things I've noticed is that the initial delays both occurred when sending messages to another T-Mobile Samsung user with RCS support. All messages sent since the initial reset have been to other carriers or non RCS devices/users. Not sure if that has anything to do with it.
There were no software changes from this morning to this afternoon; no updates, no new apps, or any other visible changes.
Other thing noticed is that I was having problems with application installations or updates from the Play store getting stuck on 100% on the download and then not finishing installation for several minutes, and in some cases never at all. This was prior to the factory reset and was fixed with the reset. I began experiencing the same issue again this afternoon around the same time as the text message issue.
I am in an area with full service bars and 4G service. It occurs at home and at work, 40 miles apart. Soft resets, cache clearing, app cache/data clearing, don't seem to work.
Next step is getting a hold of T-Mobile.