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You probably have a lot more texts on other conversations which is causing the app to lag.
I'm trying to help you here, its not quite the phone but the app, you can contact Sony regarding this >>> Local support
Hi @Aqibsh,
@uliwooly is quite right about the number of messages causing lag. It's got nothing to do with the hardware of the phone, but the more messages you have the longer a phone is going to take to load them. It's not unique to your phone either and another messaging app is going to experience the same issue.
The best way to fix this is with prevention, by occasionally deleting messages. Currently there is no auto delete function in the default messaging app that will automatically delete messages when you get to a certain limit, but there will be third party messaging apps that can do this. For the time being though, you will have to wait while it deletes the large message chains. The more you delete, the better it will become.