Many college students drop out of STEM majors after struggling in gateway courses, in part because these courses have large time demands. The risk of attrition is higher for those from financially disadvantaged backgrounds who often work to pay for college, making such time commitments unfeasible. In two laboratory experiments with different topics (central tendency and linear regression), we identified a promising approach to increase the efficiency of STEM instruction. When we removed instructional videos and taught participants exclusively with practice and feedback, they learned 2-3 times faster. However, our research also showed that this instructional strategy has the potential to undermine interest in course content for less-confident students, who may be discouraged when challenged to solve problems without upfront instruction and learn from their mistakes. If researchers and educators can develop engaging and efficacy-building activities that replace lectures, STEM courses could become better, more equitable learning environments.