This bardic masterpiece says
You can create one 10-foot-square wall per use of this ability; over several rounds you can extend this square or create other walls, all of which persist until you stop using bardic performance rounds to sustain them. ... Once created, the wall exists until you end your performance or affected creatures “break” it. Abilities that extend the duration of a bardic performance (such as Lingering Performance) affect this masterpiece.
Use: 1 bardic performance round per round.
Action: 1 standard action per 10-foot square.
Does this cost 1 performance round per round, or 1 performance round per square per round?