In situ stress measurements during molecular-beam epitaxy growth of InxGa1-xAs/GaAs provide insight into the relaxation behavior of thin films grown on mismatched substrates. Strain relaxation in the materials studied occurs due to the formation and glide of dislocations. Measurements of additional relaxation during growth interruptions show that relaxation is kinetically limited during the early stages of growth. In thicker films, the residual strain is not significantly affected by growth conditions. We find that relaxation in thick layers does not proceed as far as predicted by models which consider the formation of dislocation arrays or dislocation blocking, and we discuss this discrepancy between theory and experiment. We discuss the effect of a nonuniform dislocation array configuration on the overall relaxation behavior. Results from dislocation dynamics simulations are presented which provide evidence that nonuniform dislocation array distributions limit the extent of plastic relaxation.