  The implications of a smaller than unity speed of sound of the scalar
perturbations during cosmic inflation is studied in terms of a model
independent large-$N$ approach. We show that the $(n_s, r)$ plane is
non-trivially degenerated when the effects of a different than one speed of
sound $c_s$ are taken into account. We also discuss the degeneracies due to
$c_s$ in the running of the spectral index, $\alpha_s$. We use the bounds on
$c_s$ to constrain such degeneracies.
