On the topic of speed of focusing. In a zoom you have more options, thus, it takes more time. First you have to point it in the right place. The designer then can look at it and say: bigger or smaller. Then you have to set an edge (sharpness), then do your cuts. With a fixed focus unit all you do is point and set the edge and cuts, no extra steps, as long as the designer did his math when he drew the plot.