We measured the thermal resistance (R = 20.1 K/W) and the electrical power dependence of the electronic temperature (Re = 27.0 K/W) of THz quantum-cascade lasers (QCLs) with bound-to-continuum active region scheme in the lattice temperature range of 30–100 K. This information, obtained from the analysis of microprobe photoluminescence spectra for QCLs operating at 2.5 THz, gives an electron-lattice energy relaxation rate (0.1 ps−1) ∼ 50 times lower than THz QCLs with resonant-phonon active region scheme.