Oh , nice question. I would be quite fond of it , i love speedpainting videos. It could even motivate me to create my own. However , dont add them on the movies section. Make it a sub-section of art. Like , give the opportunity to add a speedpainting video when you upload an artwork. Then , if the watcher wishes , he can click a button on the artwork's description and see the video.
Speedpainting videos are mostly considered by people who are already into art. The majority of the members would be annoyed to have them around their favourite flash movies. And besides , a speedpainting is not a directed movie. Its just the process of creation of an artwork. Its amazing and most aspiring artists must watch as many as possible but still , not a mainstream product. Besides if the artwork is not good enough , whats the point of a speedpainting video of it in the portal?
So yeah , i think the best way to do it is to add an option for us , whenever we upload an artwork to include a speepdainting video , like the way we add tags and description. This way i can see the speedpaints i am interested in without having to swim through a sea of unwanted stuff to find them.
Furthermore , this way you keep the balance. Right now you have movies , art and music. If one starts flooding into the other , you will have issues and blurry lines. Who wants to deal with that?
Hope i helped , a little bit even. Cheers!