Assuming your Mars is FM or PCM adding a spectrum will actually DECREASE response time. You will have the benefit of no frequency conflicts and such but being used to that you may not be happy with the slight lag the Spektrum will have. I use an older Futaba 3PJ PCM/FM radio and if I use somebody's Spektrum radio the lag is noticeable to me - but no to them.
Really, ,the processing speed of the functions of the radio - such as expo, rates, etc, etc - are built in. The other factor of processing speed is in the Spektrum module and that has no relationship to the radios functions with regard to speed.