💬 In plain words: Before-save updates in Salesforce are entirely free from a performance standpoint. Because the record is already open in memory, you are simply writing on it before it gets saved. After-save updates, however, force the system to re-open the record and run the entire save process all over again, essentially doubling your resource cost. The golden rule is simple: if you are changing the exact same record that triggered the flow, do it before the save. If you are updating related records, do it after the save.
Salesforce Platform & Administration