This is worth a try, but I have my doubts if it will work. When Cubase renders down a just a handful of tracks, it uses the same amount of processing as it were rendering down all the tracks in a session. So I wouldn't expect any CPU benefit from it. Again, it's still worth trying.
If it is indeed a CPU issue, I think freezing would be worth a try particularly if you are using any VSTi's.
Brandon