Defects in polycrystalline halide perovskite films can cause a decrease of the solar cell photoconversion efficiency and stability. The perovskite film enhanced during crystal growth by controlling the processing method can alleviate defects and the related recombination sites that affect the performance of cells. Herein, flash infrared annealing is employed to crystallize methylammonium lead iodide perovskite with a single heating pulse, where uniform grain domains are optically observed and mapped. Films are annealed with different temperature ramps up to 48 °C s−1 heating rate. Annealing with higher heating rates presents lower defect densities, decreases the Urbach energy tail, and improves the optoelectrical performance of the films. These improvements are rationalized by Raman spectroscopy of nucleation points and grain surface differences among the process variations. The role of crystal growth and subsequent film quality allows to achieve a champion photovoltaic device growth at 48 °C s−1 with stability around 250 h under 1 sun illumination and 60% relative humidity for 100 h under 3 sun (AM1.5G) illumination. In situ optical imaging is recorded during the process, confirming that rapid annealing, i.e., higher heating rates, contributes to obtain more stable devices with the added advantage of shorter processing time.