A port-of-entry application can be decided the same day if the file is complete, so the constraint is usually getting the credential and a correct employer letter together, not government processing. The I-129 route takes longer and the employer controls it.
Sequence the steps the applicant does not control
Work backward from the day the applicant needs to start. The employer must produce a letter that names the profession and describes the real duties, which often takes several drafts because human-resources staff write job descriptions for recruiting, not for immigration. The credential must be located in original form, and a transcript may need to be ordered from the institution. If the applicant intends the port-of-entry route, book travel only after the letter and credential are final. If the employer prefers to file Form I-129, ask for the filing date and whether premium processing will be used, then treat the answer as the earliest possible start. Hypothetical example: an East Hants applicant accepts a start date, gives notice to a Halifax employer, and only then learns the university needs three weeks to issue a transcript. Resignation is the last step, not the first.
Hypothetical example: an East Hants wind-turbine blade engineer has an offer beginning after a U.S. project's commissioning date. The first review should identify the one document on the critical path, such as an original degree or a corrected employer letter, and assign a person to obtain it. Put a buffer between receipt of that record and any resignation, lease termination, or cross-border shipment. If the start cannot move, the employer must decide whether another lawful staffing plan is needed instead of asking the applicant to begin early.