Flashback Query is an online operation. Concurrent DMLs and queries from other sessions are allowed while an object is queried inside Flashback Query.The speed of these operations is unaffected. Moreover, different sessions can flash back to different Flashback times or SCNs on the same object concurrently. The speed of the Flashback Query itself depends on the amount of undo that needs to be applied, which is proportional to how far back in time the query goes.