What are common explanations for the angel being delayed for 21 days in Daniel 10:13?

In the book of Daniel chapter 10 , Daniel is in a period of mourning because a great war is coming. He fasts for 21 days and then sees a vision of what I presume to be an angel. Before the angel explains what will happen to Daniel's people, the angel explains why he is "late" in meeting him: > 12 Then he said to me, "Do not be afraid, Daniel, for from the first day that you set your heart on understanding this and on humbling yourself before your God, your words were heard, and I have come in response to your words. > > 13 "But the prince of the kingdom of Persia was withstanding me for twenty-one days; then behold, Michael, one of the chief princes, came to help me, for I had been left there with the kings of Persia. > > —Daniel 10:12–13 , NIV It seems very unusual to me that an angel would be delayed for three weeks by any force, and that Michael (the archangel?) had to come to help free him from the "kings of Persia" (demons?). I suppose I've always had the image that an angel could just poof to a place instantaneously, as usually happens in popular media, so being stuck in a location for three weeks and needing a stronger angel to help seems really bizarre to me and doesn't seem to match up with depictions of other angels of the Bible (e.g., the ones that appeared to Mary and Joseph). I'm really at a loss for how to explain this. What is an overview of the most common explanations for the angel being delayed here in Daniel?