Instant Pot operates at 11.6psi. How fast food cooks depends on the temperature of cooking. The cooking temperature of 15psi pressure cookers is 121ºC (250ºF); whereas, the operating temperature of an 11.6psi Instant Pot is 117°C (242°F). This is a difference of 3.4%.
We find in most cases recipes for 15psi pressure cookers can be adapted to Instant Pot by increasing the cooking time by 7-15%.