Marine heatwaves (MHWs) are increasing in both intensity and frequency against a backdrop of gradual warming associated with climate change. In the context of MHWs, animals are likely to experience sub-lethal, rather than lethal effects, defining long-term limits to survival and/or impacting individual and population fitness. This study investigated how functional sub-lethal limits track critical thresholds and how this relationship changes with warming rate. To this end we monitored basic functioning, specifically the ability to right, feed and assimilate energy, as well as oxygen consumption rate in the common Antarctic sea urchin, Sterechinus neumayeri. Water temperature in experimental systems was increased at rates of 1oC day-1, 0.5oC day-1 and 0.3oC day-1, in line with the characteristics of MHW events previously experienced at the site where the study urchins were collected on the Antarctica Peninsula. Functioning was assessed during the simulation of MHWs and sub-lethal limits determined when the rate of functional degradation changed as temperature increased. Results suggest that thermal sensitivity varies between the key biological functions measured, with the ability to right having the highest thermal threshold. Arguably, the most interesting result was that functions deteriorated at lower temperatures when warming was more rapid (1oC day-1), contrary to lethal critical thresholds, which were reached at lower temperatures when warming was slower (0.3oC day-1). MHWs and their impacts extend far beyond Antarctica and in this context, our analyses indicate that the onset rate of MHWs is critical in determining an organism’s ability to tolerate short-term elevated temperatures.