I installed the new htc messaging client 2014 on my Blakcstone (Touch HD) and with approx. 300 sms on my system and it works very normal, not slower than before without HTC messaging. Before a next upgrade I will restore all my sms (more than 1200) to see if the speed is still the same.
One remark. The first time after I restored my messages with Pimbackup it took some time to rebuild the conversations, but after that everything works normal.