Olivine-type phosphates LiFePO4 and NaFePO4 are among the most widely studied cathode materials for rechargeable batteries. To improve their rate behaviour for future electronic and vehicle applications, it is vital that the Li+ and Na+ conductivities be enhanced. In this study, atomistic simulation methods (including molecular dynamics) are used to investigate the effect of lattice strain on ion transport and defect formation in olivine-type LiFePO4 and NaFePO4, as these properties are directly related to their intercalation behaviour. The results suggest that lattice strain can have a remarkable effect on the rate performance of cathode materials, with a major increase in the ionic conductivity and decrease in blocking defects at room temperature. Such understanding is important for the future optimization of high-rate cathodes for rechargeable batteries, and is relevant to the growing interest in developing thin film solid-state batteries.