Hmm, what I'm thinking of would require some high end video editing program, but you could render the large scene as a still and then render just the small little animated part. Then you could nest the two together in a video editing program. I honestly don't know if all that work would be worth the difference in render time. Depending on your animation settings you would probably also see some difference in the animated area's background and the still area. If you can email me the files I could probably do this at school (have Adobe After Effects there) but I'm on break for the next three weeks.