The reliability of distribution networks can be improved by islanding of part of the network when a short-circuit fault occurs. The voltage and frequency stability during transient can be evaluated with time-domain simulations using component-based DER and load models. However, simulations with this type of models have a relatively high computational burden which constrains periodical stability analysis of microgrids. This paper proposes an aggregation methodology which significantly decreases the computation time of fault-initiated islanding simulations with component-based models, while maintaining the nonlinear dynamics. The methodology is first validated by directly comparing the original and equivalent models during large voltage and frequency transients. Secondly, a fault-initiated islanding case study of a residential distribution network is performed. For both validations, a sensitivity analysis of model parameters and voltage variations is performed. The results indicate the methodology preserves the dynamics of DERs and loads, while significantly reducing the computational time.