  We investigate how structural relaxation in mixtures with strong dynamical
asymmetry is affected by the microscopic dynamics. Brownian and Newtonian
dynamics simulations of dense mixtures of fast and slow hard spheres reveal a
striking trend reversal. Below a critical density, increasing the mobility of
the fast particles fluidizes the system, yet, above that critical density, the
same increase in mobility strongly hinders the relaxation of the slow
particles. The critical density itself does not depend on the dynamical
asymmetry and can be identified with the glass-transition density of the
mode-coupling theory. The asymptotic dynamics close to the critical density is
universal, but strong pre-asymptotic effects prevail in mixtures with
additional size asymmetry. This observation reconciles earlier findings of a
strong dependence on kinetic parameters of glassy dynamics in colloid--polymer
mixtures with the paradigm that the glass transition is determined by the
properties of configuration space alone.
