The spectral distributions of \ensuremath{\delta} electrons emitted in dissipative heavy-ion collisions represent an established tool to study the time evolution of the collision process. Especially, the yield of high-energy \ensuremath{\delta} electrons, which are measured for the first time up to an energy of 8 MeV, reflects short time scales and offers a unique possibility to examine the short deceleration phase at the beginning of the collision, which takes place within a few ${10}^{\mathrm{\ensuremath{-}}22}$ s. The \ensuremath{\delta}-electron spectra measured in dissipative collisions of Pb+Pb at an incident energy of 12 MeV/nucleon are compared with theoretical predictions using a coupled-channels formalism. The nuclear trajectories needed as input for the calculations are obtained from reaction models. Using nuclear trajectories calculated within the one-body dissipation model give a far too low yield of high-energy \ensuremath{\delta} electrons indicating that the deceleration predicted by this model is too slow. The new dissipative diabatic model, which takes into account a non-Markovian dissipation at the beginning of the collision and which ascribes elastoplastic properties to nuclear matter, is able to reproduce both the fast deceleration and the long nuclear contact time deduced from the experimental data.