I can remember either a homiletic professor or a speech coach commenting that it is much harder to prepare a five-minute talk than a 30 minute talk. He said that in a 30 minute talk it is quite possible to ramble on into infinity and still appear to have made a point. This is because by the end of the speech/sermon, people with a 20 minute attention span would tend to forget what was the point that was being made at the beginning. Conversely, because people have a fairly sharp memory for five minutes, any talk that seeks to communicate a point in such a way that it will stick, will have to prepare it in a concise and excellent manner that is also memorable.
Sadly, I have listened to more than one priest and pastor who has taken the approach that shorter is better and more easily remembered but then put together a talk that is neither memorable nor easily understood. The worst part is that they have used the idea of shorter and memorable in order to avoid doing the hard work that is required for a good sermon. Patting themselves on the back for not boring the congregation, they have instead left a congregation that is Biblically, theologically, and Traditionally uneducated.
But, you might say, are there not studies that appear to show that congregations prefer a shorter sermon? Well, yes and no. The reality is that if congregations are going to be presented with a dull, boring, and difficult to follow sermon, then they most certainly prefer a shorter sermon to a longer sermon. At least that way they will not have to endure as long as they would have to endure with a longer sermon. But, when an exciting speaker or preacher makes a presentation, then the reality is that congregations do not mind the longer talk because they do not psychologically see it as longer.
Priests are called to minister the Word and Sacrament. They are called to do both well. In order to do both well, they must study and prepare. That means they must program the time into their week to spend in study, contemplation, and prayer. Anything other than that is to diminish their calling and the worship of the Church.