The nonlinear evolution of electromagnetic instabilities driven by the interpenetration of two e−, e+ plasma clouds is explored using ab initio kinetic plasma simulations. We show that the plasma clouds slow down due to both oblique and Weibel generated electromagnetic fields, which deflect the particle trajectories, transferring bulk forward momentum into transverse momentum and thermal velocity spread. This process causes the flow velocity vinst to decrease approximately by a factor of √ 1/3 in a time interval tαBωp ∼ c/(vfl √ αB), where αB is the magnetic equipartition parameter determined by the nonlinear saturation of the instabilities, vfl is the initial flow speed, and ωp is the plasma frequency. For the αB measured in our simulations, tαB is close to 10 times the instability growth time. We show that as long as the plasma slab length L > vfltαB, the plasma flow is expected to slow down by a factor close to √ 1/3.