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Contradiction between no effective photophysical experiments and high device results causes the "hot exciton" thermally activated delayed fluorescence (TADF) mechanism to be still a controversial question. Here, the steady and transient photophysical characterization combined with theoretical calculation based on 4,7‐bis(9,9‐dimethyl‐9H‐fluoren‐2‐yl)‐5,6‐difluorobenzo [c][1,2,5]thiadiazole (2F‐BTH‐DMF), 4,7‐bis(9,9‐dimethyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazole (BTH‐DMF), and 5,6‐bis(9,9‐dimethyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5] thiadiazole (o‐BTH‐DMF) demonstrate that all the emitters exhibit TADF via reverse intersystem crossing (RISC) from "hot exciton" triplet excited state. The fast RISC process "hot exciton" mechanism affords a very short delayed lifetime (1 µs). Organic light‐emitting diodes (OLEDs) based on these emitters exhibit high exciton utilization over 25% and the best device shows a maximum current efficiency of 31.02 cd A−1, maximum power efficiency of 27.85 lm W−1, and external quantum efficiency of 9.13%, which are the highest performances for reported OLEDs with "hot exciton" mechanism. The experimental evidence for fast RISC process via "hot exciton" triplet state and short delayed lifetime highlights the TADF emitters with "hot exciton" mechanism for high‐performance OLEDs with very low efficiency roll‐off. The "hot exciton" thermally activated delayed fluorescence (TADF) compounds BTH‐DMF, 2F‐BTH‐DMF and o‐BTH‐DMF are designed and proved by steady and transient photophysical characterization combined with theoretical calculation. Organic light‐emitting diodes (OLEDs) based on BTH‐DMF (4,7‐bis(9,9‐dimethyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazole) exhibit high exciton utilization over 25%, high device performance, and low efficiency roll‐off, which are highest performances reported for "hot exciton" OLEDs. [ABSTRACT FROM AUTHOR]