Controlling crystallization kinetics is key to overcome the temperature–time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 Tm to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge2Sb2Te5 with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature Tg. Moreover, highly unusual for glass‐forming systems, crystallization at conventional heating rates is observed more than 50 °C below Tg, where the atomic mobility should be vanishingly small. Combining kinetic, thermodynamic, and microscopic measurements of crystallization kinetics in the classic phase‐change material Ge2Sb2Te5, it is demonstrated that Ge2Sb2Te5 crystallizes from the glassy phase at heating rates up to 10 000 K s−1 and from the undercooled liquid at higher rates. Due to the concurrence of emerging glass transition and crystallization, the activation energy of crystallization drops by more than fourfold.