I don't think 15 seconds is all that short a time, especially if you have an extension per rack at your disposal. 30 is fine, too, but equating taking 15 seconds over a shot to knowing nothing or just playing on instinct is an exaggeration.
For your standard shots, yes, they are usually done in 15 sec. or less. The problem arises with extended safety play. At the upper levels, one mistake, one edge of a ball, can easily mean loss of game. Faced with multiple safeties in a game, which is not the norm, but DOES happen, one extension is just not enough. Some shots require some thought put into them, some even being actually measured out (which even SVB does on occasion). That takes more than 15 sec. to do.
So, in those games, you are limiting the players actual potential to make a great shot or safety in favor of just banging at the ball in the hopes of staying within the time frame and hopefully getting a good outcome from it.
30 sec with an extension seems about right. 15 seconds, you WILL have some games that the clock affects the outcome of the game, and that should never happen. The game should be decided by skill, not time.