Photothermal effects can alter the response of an optical cavity, for example, by inducing self-locking behavior or unstable anomalies. The consequences of these effects are often regarded as parasitic and generally cause limited operational performance of the cavity. Despite their importance, however, photothermal parameters are usually hard to characterize precisely. In this work, we use an optical cavity strongly coupled to photothermal effects to experimentally observe an optical back-action on the photothermal relaxation rate. This effect, reminiscent of the radiation-pressure-induced optical spring effect in cavity optomechanical systems, uses optical detuning as a fine control to change the photothermal relaxation process. The photothermal relaxation rate of the system can be accordingly modified by more than an order of magnitude. This approach offers an opportunity to obtain precise in situ estimations of the parameters of the cavity in a way that is compatible with a wide range of optical resonator platforms. Through this back-action effect, we are able to determine the natural photothermal relaxation rate and the effective thermal conductivity of cavity mirrors with unprecedented resolution. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement