We investigate the response of a photonic gas interacting with a reservoir of pumped dye-molecules to quenches in the pump power. In addition to the expected dramatic critical slowing down of the equilibration time around phase transitions we find extremely slow equilibration even far away from phase transitions. This non-critical slowing down can be accounted for quantitatively by fierce competition among cavity modes for access to the molecular environment, and we provide a quantitative explanation for this non-critical slowing down.