Basically a 'quick' set is known as 'first' tempo, and is generally shorter and tighter to the net. The purpose is indeed to make it tougher for blockers to close and seal the block. For the most part, middles will always approach for a first tempo ball. If they prove proficient at executing the first tempo sets, they will almost always be able to hold (freeze) the opposing MB, who will have a hard time getting outside to close a block on a first or second tempo set to the OH/RS.
A perfect pass will always make a first tempo set easier to execute, and often - upon realizing that they will get a 'free' (non-attacked) ball, all three hitters will assume a first tempo set is coming.
For the most part, moon balls (third tempo) are considered an "out-of-system" set, usually because a poor pass makes setting anything quicker more difficult/risky. Of course, if you have a Hooker or an Eckerman, you know that even though the block will be set, they have the ability to hit over/around, or tool the block.
Most teams will be forced to adjust their defense for a proven 'faster' offense.
