[Context] In Agile software development, requirements are frequently represented in lightweight artifacts such as user stories (USs). To support their validation, USs may be complemented with additional artifacts such as acceptance criteria and test cases. [Problem] Existing studies propose frameworks for US writing, or investigate high-level factors that contribute to productivity, such as teamwork and support from management. However, empirical evidence on the impact of requirements artifacts on team efficiency is scarce. [Method] Through a mixed-methods case study at a large Dutch organization, we investigate how the use of requirements artifacts affects efficiency. We collected data from eight Agile teams: 1,345 USs from nineteen sprints, US quality indicators, the quantity of associated acceptance criteria and test cases, and cycle time. We statistically analyzed the existence of correlations between these variables, and we triangulated our results through interviews with product owners and scrum masters from the involved teams. [Findings] Our employed indicators for US quality do not seem to have an effect on on-time completion, with our interviewees highlighting the trade-off between investing on writing quality and short-term efficiency. We found, instead, that the existence of acceptance criteria may be a relevant factor for on-time completion, as confirmed both by statistical tests and the interviewees.