A few thoughts on how to reduce the framerate in your special case:

A. Simulate an open window

1. Make a screen shot or few from the street's perspektiv through a window.
2. Close the window behind the window frame with a levelblock.
3. Make a plain model with the screenshot as skin.
4. Place it before the block.
5. And attach an action to the model that manages to shift the skin relating to the cameras position, so that the skin simulates the insight through the open window.

B. Place curtains, waving in the wind

1. Place a levelblock behind the windowframe.
2. Give it a dark texture with the perspectively distorted texture of the ceiling.
3. Place a curtain with animated waving in the wind.
4. Give it a random animation action.

Last is only interesting, when you won't have the wind to be calm because of atmosphere issues.