The development of a rapid and reproducible crystallization route for the synthesis of polycrystalline metal–organic framework (MOF) films is attractive for the scalable production of nanoporous membranes on porous supports. Prior crystallization studies have primarily focused on heterogeneous nucleation, and consequently, the time-consuming growth step has been overlooked. Here, we report a crystallization using sustained precursors (CUSP) route that maintains a high precursor concentration in the growth step, hindering the undesired Ostwald ripening observed in the late stage of growth. As a result, well-intergrown polycrystalline MOF films hosting a uniform grain size and a thickness of a few hundred nanometers could be obtained at room temperature in just a few minutes. Attractive gas separation performance is obtained from ZIF-8 membranes grown in 8 min with H₂/C₃H₈ selectivity of 2433 and C₃H₆/C₃H₈ selectivity of 30. The versatility of this approach is demonstrated by synthesizing a ZIF-67 membrane, as well as for the first time, a sub-1 μm-thick continuously intergrown ZIF-90 membrane, also in a few minutes, yielding H₂/CH₄ and H₂/C₃H₆ selectivities of 19.2 and 107.1, respectively. Such advances are expected to bring the scalable production of the high-performance polycrystalline MOF membranes a step closer to reality.