In theory, it might seem that skimming could add some degree of efficiency, but in practice this is not the case. In fact, this approach actually reflects a fundamental misunderstanding of the nature of the Reading Comprehension section.
Skimming might be sufficient to absorb lighter materials, such as newspapers or magazines, but that is because those types of materials are written with simplicity in mind. A newspaper editor wants readers to know half the story by the time they have read the headline, and magazines put the most attention-grabbing pictures on their covers; these publications are trying to draw you in, to entice you to make a purchase. The makers of the LSAT, on the other hand, are well aware that they are dealing with a captive audience; they do not feel any pressure to entertain (as you may have noticed), and passages are chosen based on completely different criteria.
For many, skimming is a natural reaction to a time-constrained test, but unfortunately the test-makers are well aware of this tendency—the passages they use are chosen in part because they evade quick and simple analysis. In practice, the time “saved” on the front end skimming a passage is more than lost on the back end. In the question section, the skimmer invariably finds the need to go back and re-read, and is often not sufficiently familiar with the passage structure to locate relevant reference points quickly.
Excerpted from the PowerScore LSAT Reading Comprehension Bible