ZkinandBonez said:
Luke lost yes, but he still gave Vader a run for his money, even to the point of wounding him slightly.

What? No. Vader trolled him the entire fight. As soon as Vader’s shoulder was hit by the lightsaber, he dispensed with the pretense and cut off Luke’s hand within seconds. He could have killed Luke immediately after he walked into the freezing chamber if he had wanted to.

ZkinandBonez said:
Also, there’s two-ish years between ANH and ESB, in which he only had himself to train with. He had no practical fighting experience until he faced Vader. And there were only a few months that passed between ESB and ROTJ.

Three years between Empire and Jedi.

And regardless of the time interval, the exposition was clear that Luke’s power had grown immensely between the two films.