Before revision:  In the high-friction regime (\eta\gtrsim10^{-5}\,Pa \ensuremath{\cdot s}), for both HP and PK, k_Fs decrease as 1/\eta whereas in the low friction regime k_\mathrm{F increase as \eta increases, leading to a maximum folding rate at a moderate viscosity (\sim10^{-6}\,Pa \ensuremath{\cdot s}), which is the Kramers turnover.
After revision:  In the high-friction regime (\eta\gtrsim10^{-5}\,Pa \ensuremath{\cdot s}), for both HP and PK, k_Fs decrease as 1/\eta whereas in the low friction regime , k_\mathrm{F increase as \eta increases, leading to a maximum folding rate at a moderate viscosity (\sim10^{-6}\,Pa \ensuremath{\cdot s}), which is the Kramers turnover.