Why do many top bankers use fast speed?
With faster speed, there is much less variation from table to table and from shot to shot. There are many variables that affect how a ball banks (cloth friction, cushion friction and coefficients of restitution, OB spin transfer, and table roll-off). At faster speed, these variables don’t change the shot as much with slight changes in speed as they do with similar speed changes at slower speeds.
Also, at fast speed, the OB will pick up less forward roll into the rail (as compared to a slower bank, especially with the OB farther from the rail), and this (combined with the fast post-rebound speed) will minimize how much the OB curves forward after rebound (as compared to a slow-roll bank). As a result, the ball will bank in a more consistent direction. At slower speeds, the path and final target of the rebounding ball varies a lot with small changes in speed; unless the OB is very close to the cushion, in which case it doesn’t develop forward roll into the cushion, even at slow speed.
A disadvantages of faster speed is the reduced effective “size” of the pocket. For example, a slow-rolling ball is accepted by a corner pocket much more easily than a fast ball, especially if it doesn’t have complete rolling topspin. However, the advantages listed above more than overcome this limitation. That’s why most top bankers use fast speed on most bank shots (except where a two-way shot with pocket speed is a good play). Faster speed definitely makes banks higher percentage for people who have put in the practice time. Although, some people can be less accurate and consistent with faster speed shots, so there is a trade-off here.
For banks into a corner pocket at shallow angles into the rail, using faster speed can also help make the angle into the rail even more shallow (since the ball banks short). This can help the corner pocket accept the ball more easily (especially if the ball has some topspin into the pocket), but this is a small effect compared to the others. And for banks into a side pocket, since the effective size of a side pocket is larger at steeper angles (more perpendicular to the rail) and faster speeds, banking short with fast speed helps enlarge the effective size of the pocket; but again, these effects are very small.
Faster speed can also help one avoid an obstacle ball or reduce the chance for a double kiss. For demonstrations, see double kiss detection and avoidance. And for more info, see kick and bank shot effects.
Also, there are reliable systems for aiming fast-speed bank shots.
Now, sometimes soft speed is required for position or for two-way defensive play (e.g., in one pocket), so it is important to know how to aim soft-speed banks also (e.g., using the through-diamond rolling-ball 2-to-1 one-rail kick and bank shot aiming system).