A fiber-optic pump-probe setup is used to demonstrate all-optical switching based on intersubband cross-absorption modulation in GaN/AlN quantum-well waveguides, with record low values of the required control pulse energy. In particular, a signal modulation depth of 10 dB is obtained with control pulse energies as small as 38 pJ. Such low power requirements for this class of materials are mainly ascribed to an optimized design of the waveguide structure. At the same time, the intersubband absorption fully recovers from the control-pulse-induced saturation on a picosecond time scale, so that these nonlinear waveguide devices are suitable for all-optical switching at bit rates of several hundred Gb/s.