 Solution

Factor the momentum dependence intoWrite $s=+1$ for the intended target, obtained with the Hamiltonian sign repair in part (i), and $s=-1$ for the literal stated Hamiltonian. The same sign choice applies to both limits and cancels in their amplitude ratio. The primordial bispectrum contribution is $s{H}^4(ϵ+1-c_s^2)\mathcal{G}/(16{ϵ}^2{c}_s^4)$. HenceAt fixed $H,ϵ$ and momentum triangle, the ratio to the unit-sound-speed amplitude isKeeping only the leading slow-roll order makes this $100/c_s^4$. To compare the two pieces of this same interaction at a fixed $c_s$, their ratio is instead $(1-c_s^2)/ϵ\simeq 100(1-c_s^2)$. Thus the noncanonical piece overtakes the slow-roll piece once $1-c_s^2\gg 0.01$, while both have the common raw $c_s^{-4}$ enhancement from the interaction and its external mode normalization. For instance $c_s^2=0.1$ gives an exact ratio $9100$ to the canonical limit with these fixed parameters.

A raw primordial bispectrum amplitude is not the same as normalized primordial non-Gaussianity: here $P_{\zeta }(k)=H^2/(4ϵc_s{k}^3)$, so the ratio $B/P_{\zeta }^2$ scales as $(ϵ+1-c_s^2)/c_s^2$ at fixed shape. The small-sound-speed enhancement in normalized non-Gaussianity is therefore $c_s^{-2}$. These are tree-level weak-interaction predictions; approaching arbitrarily small $c_s$ eventually requires checking the perturbative regime.