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 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, and is relevant to the growing interest in developing thin film solid-state batteries. In addition, complementary simulation work was carried out to compare the olivine and maricite forms of NaFePO4, specifically their defect and Na-ion conduction properties.