Issue with my message folder

Hi there

Following my completion of the recent update on my old iPhone 5 v. (8.4) I started experiencing some issues accessing my message folders. The major one is that it operates really slow and i have lost the history of some of the users I am in contact most of the time. Once I press on anyone message, it takes long time to get into it and see previous text messages. On the other side, although i set that all messages should be kept all the history is not saved any longer.

However, I thought it is due to the fact the phone is quite old and i got a new iPhone 6 (16GB). Respectively, I moved my latest back-up to my new device after updated its version to (8.4) as it was (8.3). Unfortunately, the issue stays and i cannot understand what it causes it. The minute I try to get into the message folder it becomes really warm at its back side and the battery level rapidly decreases. After the reboot of it and no entry to the message folder, keeps it cool and nice.

I went into "usage'' folder then "storage" and checked out the messages weight - 399MB. Overall it says available 5.3 GB and 6.6GB used. I deleted most of the old messages and set that it keeps all messages for 30 days but the problems stays.

Any ideas how the problem could be resolved?