While the films don’t explicitly state that Vader’s suit made him weaker compared to his younger self, the idea isn’t hard to believe.

After his battle on Mustafar, Vader suffered catastrophic injuries — losing his arms, legs, and being severely burned. In Revenge of the Sith, we see that the process of placing him into the suit involved a highly invasive and complex surgical procedure. Furthermore, when Vader first stands up, we can clearly see that he struggles with basic movement and has difficulty walking. This strongly suggests that the suit restricts his mobility, preventing him from being as agile or physically capable as he was before the suit. Vader’s menacing appearance in the suit may give the impression that he is all-powerful, but this doesn’t necessarily mean he’s the most dominant force in the galaxy. Often, things that appear imposing or invincible aren’t as indestructible as they seem, and Vader’s intimidating presence doesn’t automatically equate to unparalleled strength.

I’ve never had an issue with the fact that the duel between Vader and Obi-Wan in A New Hope feels completely different from their fight on Mustafar. I’ve always chalked it up to external factors, mainly the limitations of special effects at the time, rather than trying to find an in-universe explanation for the contrast between the two duels. The difference never bothered me because I understood it was more about the practical constraints of filmmaking, rather than something that needed to be justified within the story itself.