Organic light-emitting materials with multi-resonance thermally activated delayed fluorescence (MR-TADF) have shown great potential for realizing highly efficient narrowband organic light-emitting diodes (OLEDs). However, the heavy efficiency roll-off caused by the slow reverse intersystem crossing (RISC) process remains a challenging issue for the further practical application of MR-TADF materials. Here, we develop two TADF emitters, BNDBT and BNDBF, in which the dibenzothiophene and dibenzofuran substituents are attached at the bottom of the B/N frameworks. They all exhibit the similar high photoluminescence quantum yields of 90% and 87%. The sulfur-containing material BNDBT exhibits a RISC rate (kRISC) of 6.02 × 104 s-1, which is three-folded higher than BNDBF (2.09 × 104 s-1) without heavy atom. The corresponding green OLED based on BNDBT exhibits an improved external quantum efficiency of 35.5% and lower efficiency roll-offs at high brightnesses of 100 cd m-2 and 1000 cd m-2, respectively. In addition, the BNDBT-based OLED maintains high color purity without causing a sharp increase in FWHM as compared with that of BNDBF. This work indicates that introducing the heavy atom at the bottom of the B/N skeleton is an effective strategy to enhance kRISC while maintaining narrow FWHM, thereby achieving high-performance MR-TADF emitters.