None.
Yes. Foreign judgments are routinely recognized in the United States through statutes enacted in each state. The defenses to recognition are limited in these proceedings. This principle originated with the Supreme Court’s 1895 decision in Hilton v. Guyot, and has since been codified in various uniform acts that have been adopted by the states in different forms.
No. Foreign judgments are generally enforced under uniform acts, which have limited defenses. Depending on the defenses raised, this could require discovery and could extend the timetable, but the defenses to enforcement are generally difficult to establish.