Reverse intersystem crossing (RISC), if efficient in the ambient temperature, is an effective route to harvest triplet excitons with high internal quantum efficiency in TADF-based organic light-emitting diodes (OLEDs). However, the relatively long time-scale of the RISC in typical TADF emitters is regarded as a big hurdle in suppressing or at least mitigating the device efficiency roll-off at high brightness level. To cope with such issues, we explore a system that shows a very fast RISC on the order of ca. 108 s-1 by introducing internal heavy atom effect. The study based on the proposed system unravels the relation between the spin-circulation and roll-off behavior with a strong dependence on the positive charge of the nucleus, providing a potential means to mitigate the roll-off behavior at TADF based OLEDs.