Vader is such a superior swordsman, he is playing with Luke. He was holding back the whole time until Luke lands a lucky blow and then Vader ends it by chopping off Luke’s hand.
The real question is what happened to Vader in Return of the Jedi its like be forgot how to fight. He does not fight like a great swordsman there, he looks like a frail old man. Luke is the superior swordsman almost the entire fight. Or at best they are matched and Lukes fury gives him the edge.
It’s almost as if ROTJ is an incompetently made piece of farcical
