### Rigid Body

For the rolling without sliding one has
$mgr\sin\alpha=(I_{cm}+mr^2)a/r$
that is,
$a=\frac{g\sin\alpha}{1+I_{cm}/mr^2}$
Since $v^2=2al$, where $l$ is the length of the slope, one has$v\propto \sqrt{a}$. Respectively, $t\propto 1/\sqrt{a}$.