Rounded tails, including rounded pins, squash tails, diamonds and similar variants lack the hard point of say any sort of swallow or fish tail, and thus are less likely to catch an edge when rotating. It is not exactly about fast versus less fast. In fact, speed is often the most confused aspects of a board. Loose feels fast, but is not necessarily actually moving faster.
Swallows and fish tails have more surface area, thus more area for energy to exert upon, simple physics. Additionally the cut out area provides an increased release, thus they are considered to have a feeling of being looser. However, the points of deeply cut swallows are thought to work very much like an additional fin, providing stability and direction, and is exactly why you see them so much on twin fin boards.
'fang tail'; 'toad tail'; 'stealth swallow' or stealth tail, are all variants of roughly slamming a squash or diamond tail into a swallow tail. They probably owe their origins to the bat-tail quad craze of the late 1990s. It is in a way of trying to capitalize on both design aspects or capitalized on pivot points, release areas and overall larger surface area. We started employing them back in the comp 1 days and carried it over and refined it through the elevation, hazen and pro. the elevation design with a step deck and built in kicktail, necessitated a much wider center diamond tail, coupled with the more classic swallowtail outline towards the rail. So with a wider center area, the swallow portions towards the rail get smaller, generally speaking. Toad style tails further decrease the 'swallow' portion of the tail to the point to which they are nothing but little bumps or hips or sharp like a fang or triangle.