Would importing inside/outside a flow would have an effect on performance?

Not really in a meaningful way.

I think that's a bigger concern in Airflow where that advice is coming from. Airflow keeps orchestration and execution in the same level, and constructs all its "flows" at import time to have them ready for orchestration, and refreshes that every 30 seconds, so having that be as fast as possible is important. Prefect flows are only imported in the execution layer, so there's not a meaningful difference between import time and runtime in prefect

@Austin Weisgrau Thank you for the detailed explanation!