Studies into decision-making suggest the existence of a tension between transparent and efficient decision-making. It is assumed that an increase in transparency leads to a decline in the efficiency of decision-making processes; however, this assumption has not been tested empirically. This study provides a starting point for investigating the complex relationship between transparency and efficiency on a set of 244 European Union legislative processes between 2014 and 2019. It finds that transparency neither speeds up nor slows down decision-making processes, and that the efficiency of the lawmaking process depends on political complexity. Our results call for further systematic research into the causes and consequences of decision-making transparency. Points for practitioners In decision-making processes, transparency is widely seen as both a blessing and a curse. On the one hand, it is seen as a virtue that fosters legitimacy and participation; on the other hand, it is said to reduce decisional efficiency. However, our study of 244 European Union legislative processes shows that transparency has no effect on their duration. This calls for a re-appreciation of the effects of transparency in decision-making processes.