Perceptual speed refers to the ability to accurately (and completely) compare letters, numbers, objects, pictures, or patterns. When tested, these objects may be presented at the same time or one after another. This type of test may be included in an aptitude test battery.
For instance, online you will find many quizzes that ask the reader to locate and identify one object or number out of a large group and then measure how quickly you found it. These tests purport to be tests that identify relative intelligence, but more realistically identify a talent for locating differences. We have also all seen puzzles where the reader is asked to identify a number of differences between two pictures. How long it takes to identify these differences is a measurement of perceptual speed.