Processing speed setting affects the reconstruction speed and quality.
Fast processing takes the least amount of time, but it also requires the best possible image quality. This means that the 'Fast' setting is best for those who are confident that the photos taken for the reconstruction are more than suitable. This includes the overlap, general image quality and object coverage with room to spare.
Medium setting, the default, is usually a good choice for most cases.
Slow setting allows using poorer quality pictures and provides more coverage while taking considerably more time to finish. If the Slow reconstruction setting combined with optimize cameras setting fail – there’s a high probability that the pictures were taken inadequately for the 3D reconstruction.