In reply to mutt: Pinnacle VideoSpin is good for a free bit of software, tho I'm not sure it'll do any better then MovieMaker if dealing with lots of large raster files.
Have you resized all of your rasters to a suitable size (ie 1920*1080 or 1280*720 pixels, unless your panning across images)? If not I'd try this first. If you copy and paste the original files elsewhere first to save the originals and then resize the files and save with same names in the original location you could have a very quick fix. Possibly.