Hi,
I have over 13 years of SMS gateway and application experience, in multiple value added service provider companies worldwide. I have worked with kannel and its different applications to achieve high TPS without any hassle. Optimizing a server is not only kannel related, it will require sound knowledge in kannel, even its patches and development, along with php/mysql/linux administration as well as development. this is the common combination which companies use as a sms gateway. normally servers which are not properly configured and optimized, can become slow as you put load on traffic. i am managing millions of sms per day using similar combinations.
Regards, Salman Raza