Individuals' reaction time (RT) slopes in tasks of mental rotation have been found to be related to other measures of visual-spatial abilities, and thus are often viewed as a psychometric measure of visual-spatial abilities. The common interpretation of individual RT slopes is as a measure of the speed at which the rotation is carried out. However, electroencephalography studies have found that the process of mental rotation continues after response selection has been carried out, casting doubt on the interpretation of RT slopes as measures of the speed of mental rotation. This study made use of electroencephalography techniques to directly capture individual differences in the speed of mental rotation and assess their association with visual-spatial abilities. We found that individual differences in mental rotation speed are not related to individual differences in RT slopes. Moreover, a computation model supports an alternative explanation by which RT slopes reflect individual differences in differential tolerances for stimulus identification within mental rotation tasks. (PsycInfo Database Record (c) 2024 APA, all rights reserved).