I just tried to mix a 10-Track multistem with the latest (64 bit) OSX release.
1 MIDI drum track (1x EZ Drummer) and 4 guitar tracks (LePou Amp + LePoulin IR loader each) and the DSP performance is lagging because its 100+ %.
With another DAW the CPU is “chilling” with the same project and FX.
We generally do not like to discuss issues like this on a web forum, where feedback takes too long. Find us on IRC (#ardour or #ardour-osx) and you might get some attention.