Canadians can apply at the port of entry on the day of travel, so the constraint is document readiness rather than processing time. Plan to have the letter and credentials final at least a few weeks before the intended start.
Work back from the start date to the letter
Port-of-entry applications are decided while the applicant waits, which makes them fast but unforgiving: a missing document means a refusal and a return trip, and a refusal is recorded. Set the intended start date, then schedule the employer letter, the good-standing letter and any missing transcript so that all three are in hand with time to spare. Choose a crossing and travel at a time when you can wait without a same-day work commitment. If the employer would rather file a petition with USCIS in advance so that the applicant arrives with an approval notice, that adds government processing time and cost; premium processing is available for a fee, and the current amounts should be checked on the fee schedule. Do not resign from the current Ontario role until the admission has actually happened.
Hypothetical example: a municipal planner is expected to attend a U.S. council meeting two days after her intended entry. The first review should decide whether the original credential and signed letter are physically available, rather than assuming an emailed scan will be enough. If a record is delayed, move the meeting or use the employer's petition route; do not enter to begin professional work while the application is incomplete. The admission record is the event that permits the job to start, so build the work calendar after that point rather than before it.