Macpro 5.1 to 6.1 upgrade experiences


i wonder if anyone did an update from Macpro 5.1 to 6.1 here?

im not very happy with my 12 core 3,33 ghz 5.1 because it the whole system gets very slow and laggy once projects reach 30-40 tracks, which is far less tracks than i had expected and i wonder if that is because of the 12 cores (read somewhere that cubase can handle less cores better).

Any experiences made here?