Long RAM Previews

Hey Everyone. I just got a contract to create a bunch of mograph content for a DS network and I'm doing most of the work in Motion. I'm working in 1080 HD and am experiencing long RAM Preview renders. I have a menu board loop with a motion bg and a spinning logo with everything else stationary. It's 172 frames and is taking upwards of 4 minutes to render a realtime preview. Also, any changes I make however miniscule mean a total re render of the entire project. I'm working on an Octocore machine with 5 gigs of RAM and an AJA Kona card. Is this usual or is there something I need to look at to speed things up. I trashed prefs but didn't see any improvement in render times. Thanks for any help.

Mick Haensler
Higher Ground Media