large cpu usage when monitor button engaged

Cubase 7 .0.4
I don’t know if any one else is getting this problem , when I engage the monitor button on a channel I get some serious cpu usage on the average load meter if I engage more than 5 or 6 channels cubase 7 becomes unusable with cpu overload , I never had this problem with cubase 6 im running at around 4 ms I cant run any higher buffer size as I will have a problem with latency any help appreciated

8 core mac
Mountain lion
Ssl madi extreme
14 gig ram