Exhibiting superior safety features and low costs, solid-state sodium (Na)-ion batteries have been proposed as an attractive candidate for energy storage. However, the poor rate capability of solid-state batteries has limited their applications. In this work, an all-solid-state Na-ion battery is fabricated, delivering an unprecedented rate capability (60% capacity retention at a C-rate of 100 C with an areal loading of 1.5 mg cm-2), which far exceeds other reports so far. More importantly, it is further demonstrated that instead of the Na-ion conductivity of the solid electrolyte, the rate-limiting factors are determined to be charge-transfer resistance at electrode/solid electrolyte interfaces and lack of percolation pathways in the electrode, which can be optimized by tuning the electrode design and testing protocols.