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Abstract Non‐volatile phase‐change memory (PCM) devices are based on phase‐change materials such as Ge₂Sb₂Te₅ (GST). PCM requires critically high crystallization growth velocity (CGV) for nanosecond switching speeds, which makes its material‐level kinetics investigation inaccessible for most characterization methods and remains ambiguous. In this work, nanocalorimetry enters this “no‐man's land” with scanning rate up to 1 000 000 K s⁻¹(fastest heating rate among all reported calorimetric studies on GST) and smaller sample‐size (10–40 nm thick) typical of PCM devices. Viscosity of supercooled liquid GST (inferred from the crystallization kinetic) exhibits Arrhenius behavior up to 290 °C, indicating its low fragility nature and thus a fragile‐to‐strong crossover at ≈410 °C. Thin‐film GST crystallization is found to be a single‐step Arrhenius process dominated by growth of interfacial nuclei with activation energy of 2.36 ±  0.14 eV. Calculated CGV is consistent with that of actual PCM cells. This addresses a 10‐year‐debate originated from the unexpected non‐Arrhenius kinetics measured by commercialized chip‐based calorimetry, which reports CGV 10³−10⁵higher than those measured using PCM cells. Negligible thermal lag (<1.5 K) and no delamination is observed in this work. Melting, solidification, and specific heat of GST are also measured and agree with conventional calorimetry of bulk samples.