Two overlapping authorities support a delay suit. The Administrative Procedure Act requires an agency to conclude a matter presented to it "within a reasonable time" (5 U.S.C. § 555(b)), authorizes a reviewing court to "compel agency action unlawfully withheld or unreasonably delayed" (5 U.S.C. § 706(1)), and provides the cause of action and a waiver of sovereign immunity for the suit (5 U.S.C. § 702). The mandamus statute, 28 U.S.C. § 1361, gives district courts original jurisdiction over an "action in the nature of mandamus to compel an officer or employee of the United States or any agency thereof to perform a duty owed to the plaintiff." The two are usually pleaded together; the practical standard courts apply is much the same under either.
A threshold requirement gates every delay case: there must be a discrete, non-discretionary action the agency is required to take. The Supreme Court held in Norton v. Southern Utah Wilderness Alliance, 542 U.S. 55 (2004), that a court can compel only "a discrete agency action that it is required to take." Adjudicating a properly filed petition or application generally qualifies — the duty to decide is mandatory even though the outcome is discretionary. That distinction (decide: mandatory; how to decide: discretionary) is the backbone of a delay claim.
Courts measure "unreasonable" using the six non-exclusive factors from Telecommunications Research & Action Center v. FCC (TRAC), 750 F.2d 70, 80 (D.C. Cir. 1984): (1) whether the agency's timing follows a "rule of reason"; (2) whether Congress has provided a timetable; (3) and (4) that delays touching human health and welfare are less tolerable than delays in economic regulation, and the effect of expediting this case on the agency's other, competing priorities; (5) the nature and extent of the interests prejudiced by delay; and (6) whether there is any impropriety behind the delay — though the court need not find bad faith to hold a delay unreasonable.
The forum matters, and the honest picture is uneven. In the D.C. Circuit, Da Costa v. Immigration Investor Program Office, 80 F.4th 330 (D.C. Cir. 2023), affirmed dismissal of EB-5 delay claims, giving heavy weight to TRAC factors one and four: where the agency processes cases under a "rule of reason" and moving one plaintiff to the front merely pushes others back, courts are reluctant to intervene. Other courts have been more receptive to well-documented delays — the Sixth Circuit in Barrios Garcia v. DHS, 25 F.4th 430 (6th Cir. 2022), held that allegations of harm to health and welfare (TRAC factors three and five) can alone state a claim. Congress has said, as a non-binding "sense of Congress," that benefit applications "should be completed not later than 180 days" (8 U.S.C. § 1571(b)) — a benchmark courts weigh, not a deadline they enforce.