Cubase 6.5 slow edit !


I have this problem with my Cubase 6.5. When I edit vocals (3 tracks) waves format (meaning cut & move) and trying to arrange them in line with the zoom in, there is frequent delay that annoy me. I have an iMac 3,4 Ghz, 16 Gb Ram,AMD Radeon HD 6970M 1Gb external HDD WD 3 Tb, TCK48 so at this configuration I don’t expect to have lags when I editing three waves. Does anyone knows a solution to this problem ?