render time

I have a 2010 mac pro 2.66 w 12 cores  36 megs ram running 10.8.4 - I have 5.5 and CC installed and the video card is a nvida 570

When I render a :10 comp the render time is :40 sec and is the same in both cc and cs 5.5.  When I render a 4:00 comp the 5.5 takes 4:00 but the CC takes 17 mins . I am rendering pro res in both cases and the multi thread settings are the same in both . Both are turned on , So why is CC so much slower ?? The CC version is not 12.1 , it's 12.0

Know lots has been posted on this subject but not seeing an answer

Thanks

David