BUILDER Act of 2021 or the Building United States Infrastructure through Limited Delays and Efficient Reviews Act of 2021 This bill places limits on the environmental review of proposed major federal actions under the National Environmental Policy Act of 1969 (NEPA) and establishes deadlines and other requirements to expedite the environmental review of such actions. Specifically, the bill limits the scope of the environmental review of major federal actions significantly affecting the quality of the human environment as specified by the bill. The bill also specifies the thresholds that trigger when agencies must issue environmental assessments and environmental impact statements for proposed agency actions, outlines when agencies are not required to prepare environmental documents for such actions, and establishes deadlines to complete such documents. The bill also establishes a process for designating a lead federal agency as well as a joint lead agency, such as a state government, to supervise the environmental review under NEPA. A lead agency may allow a project sponsor to prepare environmental documents under specified circumstances. Additionally, the bill revises requirements concerning the judicial review of NEPA cases, including by establishing deadlines to file lawsuits under NEPA.