Abstract Time resolved electron diffraction is an alternative approach to FEL based X-ray experiments for the study of structural dynamics of matter on the relevant timescales. The required electron beam parameters are demanding in terms of emittance and bunch length and require the operation at charges typically well below 1pC. Moreover the energy is low – a few MeV only. The longitudinal compression of the bunches can be realized with a simple longitudinal focusing scheme in a drift. In this paper the question of what limits the bunch length in this parameter regime is addressed by means of numerical simulations. Beside emittance increasing space charge effects also rf-curvature and nonlinear compression are identified as limiting factors.