The formation process of lithium-ion batteries commonly uses low current densities, which is time-consuming and costly. Experimental studies have already shown that slow formation may neither be necessary nor beneficial for cell lifetime and performance. This work combines an experimental formation variation with physicochemical cell and solid electrolyte interphase (SEI) modeling to reveal formation-induced changes within the cells. Formation at C/2 without full discharge compared to a standard C/10 formation at 20◦C notably improves the discharge and charge capacities at 2C by up to 41% and 63%, respectively, while reducing the formation time by over 80%. Model-based cell diagnostics reveal that these performance gains are driven by improved transport in the anode electrolyte phase, which is affected by SEI formation, and by enhanced transport on the cathode side. Hence, the focus on the dense SEI layer is insufficient for a comprehensive understanding and, ultimately, optimization of cell formation. All formation procedures were also tested at temperatures of 35◦C and 50◦C. Despite often surpassing the 2C discharge capacity of the standard formation at 20◦C, these cells showed comparable or lower 2C charge capacities. This suggests a pivotal role of local temperature in the formation of large-format cells.