There are some limitations when using still image sequences in a realtime composition:
1. The pixel size should be kept at a minimum. For example if you want to use a rotating station id (like the one we provide with composition:builder) don't make a full pixel size movie where only a few pixels are used, but instead make the images as small as possible and position them using composition:builder. This is also true for transparent areas, as such areas has to be rendered as well.
2. Make sure the disk drive holding the still images is fast enough (internal SSD) to deliver the data rate needed to play out the images.