Ardour is written to take advantage of multiple cores, but currently in a way that differs from many other newer releases of proprietary DAWs. We do all audio processing in one thread (i.e. on one CPU). This design is rooted on the assumption that the user will have 1 or 2 CPUs, and thus allows dual CPU systems to have a CPU free for GUI, disk i/o, MIDI & OSC I/O etc. I believe that this was the right design until very recently.
What has changed is that 4-CPU systems are now becoming relatively affordable. With 2 CPUs, I don’t believe that the “use as much CPU as possible for DSP” is the right approach for most users. But with 4, a different model is needed.
Some version of Ardour 3.0 will feature parallelization of the audio processing code.