  Black holes of sufficiently large initial radius are expected to be well
described by a semiclassical analysis at least until half of their initial mass
has evaporated away. For a small number of spacetime dimensions, this holds as
long as the black hole is parametrically larger than the Planck length. In that
case, curvatures are small and backreaction onto geometry is expected to be
well described by a time-dependent classical metric. We point out that at large
$D$, small curvature is insufficient to guarantee a valid semiclassical
description of black holes. Instead, the strongest bounds come from demanding
that the rate of change of the geometry is small and that black holes scramble
information faster than they evaporate. This is a consequence of the enormous
power of Hawking radiation in $D$-dimensions due to the large available phase
space and the resulting minuscule evaporation times. Asymptotically, only black
holes with entropies $S \geq D^{D+3} \log D$ are semiclassical. We comment on
implications for realistic quantum gravity models in $D \leq 26$ as well as
relations to bounds on theories with a large number of gravitationally
interacting light species.
