The fluctuation-based technique has been used to shed light onto the physical processes responsible for the performance of nitride heterostructure field effect transistors. A non-monotonic dependence of hot-phonon lifetime on the density of two-dimensional electron gas (2DEG) is found and interpreted in terms of plasmon–longitudinal-optical-phonon resonance. Experimental data show that the hot-phonon lifetime acquires the shortest value in the vicinity of the resonance. At a fixed 2DEG density of the 2DEG, the resonance can be tuned in with supplied electric power. The signatures of the resonance have also been resolved in frequency performance of the transistors and their degradation. In particular, the transistor operation is the fastest and the reliability is the highest when the hot-phonon lifetime is the shortest.