We have obtained microscopic evidence of the influence of domain wall stray fields on the nanosecond magnetization switching in magnetic trilayer systems. The nucleation barrier initiating the magnetic switching of the soft magnetic Fe₂₀Ni₈₀ layer in magnetic tunnel junctionlike FeNi/Al₂O₃/Co trilayers is considerably lowered by stray fields generated by domain walls present in the hard magnetic Co layer. This internal bias field can significantly increase the local switching speed of the soft layer. The effect is made visible using nanosecond time- and layer-resolved magnetic domain imaging and confirmed by micromagnetic simulations.