This work demonstrates the importance of transport phenomena on the synthesis of halide perovskite nanoplatelets. Illustrated herein for CsPbBr3, we demonstrate the need for fast and homogeneous mixing to achieve monodispered crystals, in this case with sharp blue emission. The modularity of our synthesis method, combining fluid dynamic simulations, bespoke 3D flow reactors and on-line measurements (UV-vis absorbance and photoluminescence) reveals fundamental understanding about the growth of these perovskite nano-materials, which in turn leads to superlative size reducibility (2.2 ±0.3 nm) and properties (sharp blue emission at 472 nm wavelength, and PLQY 24.5±0.4%). The counterpart conventional hot injection batch synthesis with the same precursors, conditions and reaction temperature yields a product with PL at 501 nm with large batch to batch variations. Understanding the dynamics of the synthesis in the very early stages (within the first 100-300 ms) shows that mixing of the precursors plays a key role not only during the nucleation step, as previously believed, but also during the growth stage. Such fluid properties can be easily predicted in purposefully designed 3D microreactors to provide a consistent and steady output capable of fulfilling the large demand of blue light emitters for a variety of applications.