I always thought that Maeve doing it there & in the NeverNever during PG was a bit facile. My personal thought is that it is the Outsiders.
Here’s an interesting thought though - what if the time distortion field were caused by Demonreach? What if the time distortion field were there to slow down the prisoners from getting out should the banefire fail?
We know that Demonreach knows of the attack because one of its’ defenses is in time - there is probably some sort of defense mechanism on the island where the trigger is always a few days ahead. For example, if today is a Saturday, then DR always has an advance alarm system whose boundary extends through maybe Monday/ Tuesday. Tomorrow on Sunday, it would extend through Tuesday/ Wednesday. This way the island is always aware of attack a few days in advance - enough time for it to get help.
We also know that Odin knows a lot about the island since he was the OG Merlin’s teacher (and Merlin built the island). So isn’t it suspicious that he is able to recognize & deal with the effects by his lonesome?
Arguments against other theories
Is Maeve really powerful enough to coordinate the Fae in their attack on the Demonreach entity while holding up a time spell over a large body of water that is complex enough to require all of Odin’s efforts for the fight? Nothing we have seen from her shows her to be capable of this.
Regarding what Odin says to Harry - he only says that Harry has felt these effects before, Harry is the one who takes this to mean the events in Proven Guilty, when time was sped up, allowing Mab’s armies to reach Arctis Tor ASAP. Perhaps this time distortion was caused by the breach of Arctis Tor’s defenses (I wouldn’t be surprised if Mab had a relationship with its genius loci), speeding time up in Winter to allow Mab’s forces to come to her aid ASAP. They were there after Harry pretty soon after he rescues Molly.
