 This study presents a novel single-molecule-tweezer method which allows for the observation of multiple structural transitions of a designer single-chain transmembrane homodimer over a period of nine hours. Using this method, the authors were able to determine the energy barriers and folding times for the transitions, providing insight into the speed limits of helical membrane proteins. They found that the speed limit was much lower than previously thought, with a value of approximately 21 mls for the helix-helix interaction. This suggests that the viscosity of lipid membranes plays a significant role in slowing down the process of membrane protein folding. This article was authored by Seo Yoon Kim, Dae Hyol Lee, WC Bachini Wai Jing and others.