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. This journal is © The Royal Society of Chemistry.