Ultrasensitive thermal lens (TL) spectroscopy, where the interplay between conduction and convection is crucial, provides profound insights into molecular behavior. This work focuses on the critical role of molecular convection by using the dual-beam z-scan method and time-resolved TL spectroscopy. We specifically investigated the correlation between the detection iris and the spot size of the pump beam. Additionally, we address detection limitations and their influence on the perception of convection timing. We introduce an experimentally derived ratio optimized for higher sensitivity, enhancing the reliability of TL spectroscopy. This refined approach to TL spectroscopy allows for a deeper exploration of molecular characteristics in liquids.