Transfer time is only one component
Copying bytes may be fast while preparing trustworthy data takes much longer. Migration work can include source discovery, field mapping, cleanup, transformation rules, test loads, exception handling, reconciliation, sign-off and cutover.
The more systems and owners involved, the more coordination matters. A technically quick load can still have a long elapsed timeline because decisions and validation happen between runs.
Test with representative data
A small perfect sample can hide real problems. Test data should represent common records and meaningful edge cases without exposing information improperly. Time the full cycle, including finding and correcting defects, not just the import command.
If rollback, backup, privacy, security or regulatory requirements apply, those controls belong in the project plan and should be designed by appropriate professionals.
Plan a cutover window
Cutover timing depends on acceptable downtime, transaction volume, synchronization method and business calendar. Build the schedule around the system-specific recovery and verification plan rather than assuming the fastest transfer is the safest option.