Efficient implementation of the continuous-time hybridization expansion quantum impurity solver. Nature of problem: Quantum impurity models were originally introduced to describe a magnetic transition metal ion in a nonmagnetic host metal. They are widely used today. In nanoscience they serve as representations of quantum dots and molecular conductors. In condensed matter physics, they are playing an increasingly important role in the description of strongly correlated electron materials, where the complicated many-body problem is mapped onto an auxiliary quantum impurity model in the context of dynamical mean-field theory and its cluster and diagrammatic extensions. The quantum impurity model still constitutes a nontrivial many-body problem, which takes into account the (possibly retarded) interaction between electrons occupying the impurity site. Electrons are allowed to dynamically hop on and off the impurity site, which is described by a time-dependent hybridization function.