Before the employer's support letter is finalized, so that the duty description and the degree description can be written against records that actually exist rather than corrected afterwards. Institutional turnaround varies, so ask for an estimate at the point of ordering.
Order records before the wording is locked
If the employer letter describes duties requiring a specialization the file cannot yet evidence, either the letter has to be redone or the records have to be rushed. Requesting the transcript and any registrar letter first lets both documents be drafted from the same facts. Where the wait is long, tell counsel what exists now so the strategy accounts for the records in hand.
Work back from the first day the applicant must be on site and identify what has to be finished before then, including the employer letter and any credential documents from institutions abroad. Reserve time for the transcript to arrive, for the U.S. entity to sign a corrected letter, and for the household to settle the spouse's plan. A Canadian can generally present the finalized package at a port of entry or have the employer pursue Form I-129 where suitable. The real constraint is the days needed to correct a support letter, not the adjudication itself.