Thermally activated delayed fluorescence (TADF) emitters are attracting increasing attention because of their high electricity-to-light conversion efficiencies in organic light-emitting diodes (OLEDs). The electricity-to-light conversion efficiency of TADF emitters depends largely on their rates of radiative decay and transition from triplet to excited singlet states. To realize high-performance TADF-based OLEDs, TADF emitters should exhibit both rapid radiative decay and rapid triplet-to-singlet conversion. Herein, we describe a rational molecular design strategy for such TADF emitters based on theoretical and computational chemistry. Our design strategy provides a guideline for realizing high-performance TADF-based OLEDs.