The phrase "quick on the uptake" refers to someone who is highly perceptive and able to grasp new concepts or ideas with ease. Antonyms for this phrase would, therefore, describe individuals who are slow or struggle to understand new information. Some possible antonyms include "slow-witted," "dense," "unintelligent," "obtuse," "dim-witted," "uncomprehending," and "mentally sluggish." These terms describe individuals who require more time and effort to process new information and who may struggle to keep up with others who are quick on the uptake. Of course, it's important to remember that intelligence and ability can vary greatly from person to person, and that labels and categories are not always helpful in understanding individuals.