  We measure the quasiparticle weight in the heavy-fermion compound
CeCu$_{6-x}$Au$_{x}$ ($x=0,\ 0.1$) by time-resolved THz spectroscopy for
temperatures from 2 up to 300\,K. This method distinguishes contributions from
the heavy Kondo band and from the crystal-electric-field satellite bands by
different THz response delay times. We find that the formation of heavy bands
is controlled by an exponentially enhanced, high-energy Kondo scale once the
crystal-electric-field states become thermally occupied. We corroborate these
observations by temperature-dependent dynamical mean-field calculations for the
multi-orbital Anderson lattice model and discuss consequences for quantum
critical scenarios.
