  Rapidly rotating black holes are a prime arena for understanding corrections
to Einstein's theory of general relativity (GR). We construct solutions for
rapidly rotating black holes in dynamical Chern-Simons (dCS) gravity, a useful
and motivated example of a post-GR correction. We treat dCS as an effective
theory and thus work in the decoupling limit, where we apply a perturbation
scheme using the Kerr metric as the background solution. Using the solutions to
the scalar field and the trace of the metric perturbation, we determine the
regime of validity of our perturbative approach. We find that the maximal spin
limit may be divergent, and the decoupling limit is strongly restricted for
rapid rotation. Rapidly-rotating stellar-mass BHs can potentially be used to
place strong bounds on the coupling parameter $\ell$ of dCS. In order for the
black hole observed in GRO J1655-40 to be within the decoupling limit we need
$\ell \lesssim 22$ km, a value 7 orders of magnitude smaller than present Solar
System bounds on dynamical Chern-Simons gravity.
