The hardening effect caused by the relaxation of nonequilibrium grain boundary structure has been explored in nanocrystalline Ni–W alloys. First, the kinetics of relaxation hardening are studied, showing that higher annealing temperatures result in faster, more pronounced strengthening. Based on the temperature dependence of relaxation strengthening kinetics, triple junction diffusion is suggested as a plausible kinetic rate limiter for the removal of excess grain boundary defects in these materials. Second, the magnitude of relaxation strengthening is explored over a wide range of grain sizes spanning the Hall–Petch breakdown, with an apparent maximum hardening effect found at a grain size below 10 nm. The apparent activation volume for plastic deformation is unaffected by annealing for grain sizes down to ∼10 nm, but increases with annealing for the finest grain sizes, suggesting a change in the dominant deformation mechanism for these structures.