No single race is the "dumbest." Cognitive test averages differ by ancestral population: East Asians and Ashkenazi Jews highest (~105-110), Europeans ~100, US African Americans ~85, sub-Saharan African populations lowest (~70) in major compilations.
These are broad group statistics only. Variation within every group vastly exceeds differences between groups. Data has limits, debates on causes continue, and individuals range widely everywhere. Judge people as individuals.