Like alphadog above, I think the average may be higher than expected and not really a great measure of actual shooting pace due to some key shots requiring a great deal of thought.
Take a common example...
A pro breaks and surveys the table, mapping out the entire pattern for the out. Once complete, he gets down and starts to shoot, often getting down on the next shot as soon as he walks to the part of the table he needs to shoot from. The balls are picked off very quickly with barely any time in between them, but the average, due to the gap between the break and shooting the first ball taking as long as the rest of the rack combined, will be much higher than their actual shooting pace.
Weaker players need more time because they either don't have a plan for the full rack and need to keep thinking about the ball that comes after the next one OR even if they did make a plan, their execution leaves them short/long on position and forces them to rethink their pattern.
Personally,, I might take 1-2min to plan out a rack, then in an ideal world I stay in line and pick off the balls about 10seconds apart, if that. Things being less than ideal fairly often, I generally need to rethink my approach after a positional play forces me to and I use up one if not multiple 'extentions' in shot clock parlance over the course of the rack. So my preferred pace is quick and that is how I shoot best, but the thinking side of the game eats up my table time as I don't just glance at a table and see the pattern like some of the pros we see on TV who have played 10s of 1000s of racks more than I have (and probably saw the patterns more quickly even when they were at my level of experience.)