When SMS input gets slow don't restart the phone (as this does solves the issue). Open up the SMS app and then press and hold the home button until it closes. When you restart SMS, input will be fast again.
This, I presume, is because the "press and hold home button" method properly closes apps and thus clears out the memory usage of SMS which, I once again presume, is the cause of the sluggishness
This, I presume, is because the "press and hold home button" method properly closes apps and thus clears out the memory usage of SMS which, I once again presume, is the cause of the sluggishness