It has been recently demonstrated that co-flow water cavitation peening introduces beneficial compressive residual stresses in aerospace materials. In this process, cavitation is produced in the shear layer between two concentric co-flowing water jets with a large velocity difference. Although prior studies have shown that material response is greatly influenced by the jet velocities and standoff distances used, no effort has been made to explain the observed trends by correlating the salient features of the cavitating flow with the resultant process performance. This article investigates the effect of jet velocities in co-flow water cavitation peening through high-speed imaging and surface-pitting tests on Al 7075-T651 alloy. High-speed imaging is used to determine the cavitation cloud width as a function of the inner and outer jet velocities. Results show that the observed increase in mass loss with inner jet velocity (Vin) is due to a corresponding increase in cloud width. The high-speed imaging results also highlight the trade-off between the cloud width and the cloud occurrence probability at different outer jet velocities Vout. While a lower Vout yields a larger cavitation cloud width, a higher outer flow velocity sustains the cavitation for a larger distance, resulting in more cavitation impacts on the material surface. As a result, for the range of inner jet velocities considered, an optimum value for the outer flow velocity (Vout = 11.0 m/s) is shown to exist, which is confirmed by mass loss and strip curvature measurements. Surface micropitting analysis is used to evaluate the cavitation intensities at different flow conditions. The combined pitting and high-speed imaging results explain why higher cavitation loads are produced at the optimum flow conditions of Vin = 150 m/s, Vout = 11.0 m/s, and sn = 45.