  When is the quantum speed limit (QSL) really quantum? While vanishing QSL
times often indicate emergent classical behavior, it is still not entirely
understood what precise aspects of classicality are at the origin of this
dynamical feature. Here, we show that vanishing QSL times (or, equivalently,
diverging quantum speeds) can be traced back to reduced uncertainty in quantum
observables and thus can be understood as a consequence of emerging
classicality for these particular observables. We illustrate this mechanism by
developing a QSL formalism for continuous variable quantum systems undergoing
general Gaussian dynamics. For these systems, we show that three typical
scenarios leading to vanishing QSL times, namely large squeezing, small
effective Planck's constant, and large particle number, can be fundamentally
connected to each other. In contrast, by studying the dynamics of open quantum
systems and mixed states, we show that the classicality that emerges due to
incoherent mixing of states from the addition of classical noise typically
increases the QSL time.
