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Abstract While Nb₃Sn theoretically offers better superconducting radio-frequency (RF) cavity performance (Q₀and $E_{\mathrm{acc}}$) to Nb at any given temperature, peak RF magnetic fields consistently fall short of the ∼400 mT prediction. The relatively rough topography of vapor-diffused Nb₃Sn is widely conjectured to be one of the factors that limit the attainable performance of Nb₃Sn-coated Nb cavities prepared via Sn vapor diffusion. Here we investigate the effect of coating duration on the topography of vapor-diffused Nb₃Sn on Nb and calculate the associated magnetic field enhancement and superheating field suppression factors using atomic force microscopy topographies. It is shown that the thermally grooved grain boundaries are major defects which may contribute to a substantial decrease in the achievable accelerating field. The severity of these grooves increases with total coating duration due to the deepening of thermal grooves during the coating process.