Why are some moves instant, and some take a while?

If all your walls are created from the same geographical location, the files will be in the same data center, and moves will be instant. If you have walls in different locations, the files will disappear immediately from the source wall, then be transferred in the background and reimported on the destination wall, by different servers in a different region. This takes approximately the same time as uploading a file, photos will take a few seconds while video processing can take several minutes.