Ideal video format for Windows Movie Maker Windows Movie Maker is very slow in converting from MP4 to whatever it's using as its preferred/interim format. What is their preferred format, so I can convert to it using faster and/or GPU accelerated programs?

I've somewhat ruled out WMV, however, I converted the video to it, and WMM is processing it a good amount faster.

What is the ideal format to use for movie maker for importing video?

55 hours of video to encode/process with microsoft "technology" is not fun!
Windows Movie Maker, being a Microsoft product, works with WMV files using the Windows Media Video codecs.  You can use the free Windows Media Encoder to encode the files.  I believe there is also a free version of Microsoft Expression Encoder that you could use as well.  Expression is a bit newer, but slightly less capable since they moved a lot of functionality to the commercial Expression Encoder Pro (the Pro version is unfortunately not free).