I see Rey’s fast progress as playing catchup to Kylo. Everything she has learned as been from him. He tries to read her mind and then she reads his. He tries to use the force on her and then she uses it on the Stormtrooper. He is skilled with the lightsaber and she uses the same trick. Kylo is her teacher, not Luke. That is why she has what she needs already. Plus the patience. Her conflict is with Kylo, her equal in nearly every area. Yes, she is picking up these incredible powers, but she is facing an adversary even more powerful and practiced. I see that as her character’s main conflict. Luke’s was learning the patience to do things right. Rey has the patience and she just needs the skills and some clue how to deal with Kylo and save the galaxy. I don’t think things are as easy for her as some are making it out. The force powers are not her challenge. Kylo is her challenge. No one else who has faced him has had any impact on him. So I don’t find her fast learning of force powers to make her a Mary Sue because they only put her on equal footing with her adversary. He character journey is defined in other ways, not by the force powers she is learning. When you look at her scenes with Kylo, she has not had it easy at all. Things have not gone her way. She doesn’t have it too easy by any means.
Vader was also better than Luke in every way possible until Return of the Jedi. In the other 2 OT movies Vader beat Luke (including in Luke’s area of expertise: flying). So Luke had to overcome his adversary and become his superior in order to win, like Rey, AND learn to be patient and about the ways of the force, having to face many challenges along the way.
You say Kylo is Rey’s challenge - I respectfully disagree. She’s better than him in every single way possible. She hasn’t been bested by him at all, she in fact kicked his a**, and the second movie in a trilogy should be the character’s low point, where they suffer, where they lose. Yet Rey didn’t suffer in this movie at all. She had to face the reality of who her parents are, yes, but she never cared about that after those 1 or 2 scenes anyway.
I just wanted her to be a more compelling character, a more well developed character, because she’s extremely charismatic, and would be such a likeable protagonist, but unfortunately she’s just a bland and flat character to me.