<p>This study quantifies the welfare costs of monetary policy inertia during the 2021–2023 inflation episode in Poland and disentangles the relative contributions of parameter uncertainty and policy delay. The analysis is conducted within a country-specific open-economy New Keynesian framework calibrated to the Polish economy. Three testable hypotheses are evaluated. First, the study examines whether the actual policy rate path of the National Bank of Poland systematically deviated from a robust policy rule designed to perform under parameter uncertainty. The results indicate persistent negative deviations from the benchmark rule throughout the sample period, suggesting a consistent degree of policy inertia. Second, the paper assesses the relative welfare costs associated with policy delay and parameter uncertainty. Using a Shapley value decomposition, the findings indicate that policy delay directionally accounts for a larger share of the excess welfare loss than parameter uncertainty, with approximately 57 percent of the loss attributable to delayed policy adjustment and 43 percent to parameter uncertainty. Sensitivity analyses across alternative persistence parameters confirm the robustness of this qualitative result. Third, the performance of the proposed robust policy rule is compared with conventional policy benchmarks, including certainty-equivalent and Taylor-type rules. Simulation results show that the robust rule consistently outperforms these alternatives across policy environments characterized by uncertainty and implementation delays. Overall, the findings highlight the macroeconomic costs associated with delayed monetary policy responses in inflationary episodes and suggest that robust policy rules may provide a practical operational framework for monetary policy design under parameter uncertainty.</p>