To what extent is registration necessary or beneficial in an infringement lawsuit? What is the process of expediting registration?

While registration is generally not required to obtain copyright protection, it is currently a required administrative process to file a lawsuit. Specifically, the plaintiff in a copyright infringement lawsuit must acquire the "certificate of registration" in oder to formally file the lawsuit. The registration process is straightforward. It involves the submission of an application, a fee, and deposit materials. The normal processing of registration may take six weeks or longer. In cases of urgency (e.g. litigation) it is possible to request special expedited handling by preparing the form "Request for Special Handling" and paying an additional expedited handling fee of $500. In these cases, the Copyright Office will process the application within five working days.

Generally actions for infringement cannot be instituted until registration of the copyright claim has been made. However, there are a couple of exceptions to this rule. For instance, if the registration is refused despite having filed the correct form, the copyright owner can still institute the lawsuit. Other exceptions include simultaneous transmissions and certain situations under the Berne Amendments.

Additionally, copyright registration within five years of first publication creates a legal presumption of ownership and validity. While copyright registration does not prove ownership of copyright, this legal presumption of ownership and validity constitutes a prima facie proof (on first appearance). Consequently, in cases of infringement the court will presume that the ownership of the copyright and the registration is valid and will shift the burden of proof to the defendant to disprove by showing with sufficient evidence the falsity or invalidity of the allegation.

Finally, statutory damages can only be awarded in a copyright infringement lawsuit if the work was registered prior to the infringement or within three months of the first publication of the work.

References:
[1] Stim, R. "Intellectual Property. Patents, Trademarks, and Copyrights" West Legal Studies.
[2] Black's Law Dictionary 5th ed., (West Publishing, 1979).
[3]
Copyright Act of 1976