One of the explicit goals of the Lisbon Treaty was to enhance the ‘efficiency and democratic legitimacy of the Union’. To increase the Union’s efficiency, the treaty extended the scope of application of the qualified majority voting rule in the Council. To boost the Union’s democratic legitimacy, it also widened the scope of application of the co-decision procedure, which grants the European Parliament formal powers that are on par with those of the Council. Relying on a quasi-experimental research design and a new dataset covering all legislative decision-making processes in the period between 2005 and 2011, this study focuses on the effects of those formal rule changes on the efficiency of Council decision-making. The analysis corroborates earlier regression-based research on pooled samples that indicate that the empowerment of the EP prolongs decision-making. In fact, the co-decision procedure more than doubles the duration of Council decision-making in those areas where it was introduced by the Lisbon Treaty. However, the analysis also demonstrates that, due to the ‘limited diversity’ of real world cases, it is practically impossible to identify the causal effect of changes in the voting rule. This problem has likely affected previous research as well, but has been masked by the method of analysis employed.