In practical applications, high power is the ultimate goal of dielectric capacitors, while most research has focused on enhancing energy density. Far less attention is paid to enhancing discharge speed and achieving high power. In the present work, we studied the fast discharge behavior of (Pb0.98−xLa0.02Srx)[(Zr0.50Sn0.50)0.88Ti0.12]0.995O3 antiferroelectric ceramics and investigated the effects of phase transition on energy release rates. By doping Sr content, the antiferroelectricity was enhanced and the sharpness of the hysteresis loop was reduced, suppressing the constant and slow discharge current plateau caused by ferroelectric–antiferroelectric (FE–AFE) switching. As a result, the average power density was enhanced from 0.3 to 4.1 MW/cm3. More importantly, higher power density could be achieved in composition with lower discharge energy density but faster discharge speed. This study proves the significance of discharge speed and provides a new approach to develop dielectric materials for high-power applications.