This paper discusses stability conditions of direct torque control (DTC) based on a singular perturbation perspective. Input-output decoupling is discussed and asymptotic-input-output-decoupling is proposed in order to improve erratic DTC performance reported by practitioners. Stability analysis shows that DTC is applicable for low-leakage machines. A detailed mathematical stability analysis and an experimental comparison of conventional DTC and decoupling methods under high speed and high load condition are presented. The results show that asymptotic decoupling compensates for erratic DTC performance with minimal control effort and controller complexity. The improvement trades off voltage headroom for dynamics of torque regulation.