Despite the persistent focus on terminology in legal translation studies, to date, no large-scale research has empirically explored the difficulty of terminology in translating legal genres. Approaches to translation difficulty in translation studies more broadly remain limited in scope. To fill this gap, a study was conducted to measure the difficulty associated with the translation of legal terminology and phraseology, as well as with terminology of other domains, in the LETRINT 1+ corpus, including nine representative genres of three institutional settings (the European Union, the United Nations and the World Trade Organization). For comparative purposes, four levels of translation difficulty were assigned to multiple terminological features by a group of specialized translators through a consensus-building process of annotation based on the cognitive effort estimated for translation decision-making. The difficulty scores obtained confirm the correlation between legal singularity and higher translation difficulty, as well as the connection of more commonly used legal terms and phrasemes, and core economic terms, with lower difficulty levels. The findings also provide evidence of the prominence of non-legal specialized terminology in institutional legal discourses, and the aggregate terminological difficulty levels of each genre examined, which can be particularly useful for informing translation quality assurance, project management and translator training.