Although photoreaction quantum yields of photoswitches determine their switching efficiency, the rates of those reactions are essential parameters because they can establish the eventual temporal resolution of the device using the switch. 1,2-Bis(3,5-dimethylthiophen-2-yl)hexafluorocyclopentene (DMT) features efficient photochromic reactions of both ring-opening and closure and a markedly short time constant of the ring-opening reaction. We have found that the latter is due to the fact that the electronic relaxation from the S1 state of the closed-ring isomer of DMT occurs through a single dissipation channel, leading to a conical intersection in which the DMT molecule possesses open-ring-like geometry.