Understanding the kinetics and energetics of metal halide perovskite formation, particularly from the structural point of view at the nano-regime, is important for the advancement of perovskite devices. Rational insights are needed regarding the mechanism by which perovskite conversion reactions occurs and their kinetics. Here we examine the structural evolution of precursor and perovskite phase using in situ synchrotron x-ray scattering. This approach mitigates issues associated with illumination and electron beam-based techniques and allows fundamental conclusions to be drawn regarding the kinetics of these reactions. We find that kinetics and the orientation of grains strongly depend on both the lead halide framework and the nature of the A-cation, with fastest kinetics for MAPbI3, followed by FAPbI3, and slowest for MAPbBr3. Molecular dynamics simulations and density functional theory calculations further reveal that these reactions are diffusion-controlled with a hopping time of 5-400 s, corroborating experimental results while adding new dimensions to conversion reactions.