I've been having this same problem for at least a year. I thought it was my 6S, which was having battery problems. I then upgraded to the 8 with max memory thinking that would help - NOPE! Then upgraded to the X & dropped over $1,000 hoping to get this infuriating problem resolved - WISHFUL THINKING! I've combed through all the forums & what's even more frustrating than none of the recommendations I've read working, but that Apple has the audacity to seemingly be unaware of this problem. Given volume of texts I send, this has to be one of the most frustrating things ever & with it going on for so long, I'm stunned there's no fix!!!
This is one of 3 things over the last year that has me re-thinking my 15 year+ loyalty to all things Apple!!!
APPLE - if you read these forums, take action and/or or have a fix to this problem, please share with your development team, engineering, and genius bar personnel, none of which seem to know what the problem is. You're on the verge of losing a client who uses EVERY product.