Rewatched it.
It's a combination of Rey being all-powerful, and equally Kylo Ren losing his powers.
Till the point she is captured, you can be sure of their relative abilities in combat. We had seen Kylo display some good force powers (stopping the blaster, mind-tricking Poe, toying with Rey and making her faint). Rey managed to reverse the mind trick. That's IMO believable (maybe she was very strong and that's what happens when you resist it).
Then she managed to trick the trooper without apparently knowing what the force is. Hm?
Then in the final fight, first Kylo demolishes Rey with a force push. He wants the lightsaber from Finn. He doesn't try to use the force to take it and walk off. Hmmm.
Then he takes ages to beat someone completely untrained and not strong with the force, even taking a hit from Finn. ?
Finally she manages to fight with him evenly, which is stretching credibility, and then she slaughters him in probably the most one-sided lightsaber fight in all 6 movies. What???
Apart from that, I really liked Finn this time around. Nice character. Poe and Rey are fine too. The pace of the entire film is terrific. And the whole Starkiller Base plot is ridiculous and unnecessary.