I doubt many people make actual full-length movies with Poser 5.  As you noticed, it's too slow. 

You can make short videos, with a little planning and patience.

Consider using the P4 renderer instead of Firefly.  You may not notice the difference in a video. 

Use smaller figures - V3 RR instead of the regular V3, or even Posette.  Use smaller textures, or shaders instead of texture maps.

Render a series of still images, instead of to AVI.  That way, if the render crashes part-way through, you can just continue from where it left off.