Actually it was a test song I made to understand how applying effects/filters on grouped tracks work, and I deleted it.
I forgot to say something in my previous post. Using a bus instead of "stereo output" for 2 tracks in a test song, increased a LOT the CPU use. After doing this, I couldn't play the song more than 2 seconds without the "cpu warning" popup. Once I removed the bus intermediary (sending those 2 tracks directly to the stereo output), the song was playing fine again. So it's really a great option on the paper, but not usable for my config (mac book pro late 2011 2,4 GHz Intel Core i5, 8GB).