Does using JACK add latency?

There is NO extra latency caused by using JACK for audio input and output. When we say none, we mean absolutely zero. The only impact of using JACK is a slight increase in the amount of work done by the CPU to process a given chunk of audio, which means that in theory you could not get 100% of the processing power that you might get it if your application(s) used ALSA or CoreAudio directly. However, given that the difference is less than 1%, and that your system will be unstable before you get close to 80% of the theoretical processing power, the effect is completely disregardable.