It’s not a matter of it being “hard to believe”. I don’t like it. You can’t take a character ppl love, take away from some of what they loved about him and expect ppl to be cool with it.

I love Darth Vader, and he doesn’t have to be a god, and shouldn’t, but he should still be powerful.
I find Vader and Palpatine’s dynamic in Canon more fascinating, because his hold over Vader is actually more psychological and built on contingencies. Also, while Vader has more raw power, Sidious is more into dark side sorcery.
It fits both character’s personalities and balances their dynamic more. Vader is the brutal warrior enforcer, the Emperor is the manipulative sorcerer politician.
That’s mostly why I love the Emperor almost just as much as Vader; he’s evil yet in opposite to Vader in many ways. He’s his perfect compliment.
Anyway, Vader is allowed to have some cards on the table. So much so that Vader actually has some bartering power with Palpatine. Point being, Vader shouldn’t be wimpy.

The suit hardly restricts mobility in the OT. He does a big jump in ESB and shows some agility in dueling in ESB and ROTJ. The reason he walks that way in ROTS is because he was just put in the suit and went through surgery.
And sure, he’s not a super lean, agile fighter, but Vader makes up for it with the ferocity and power of his strikes. His suit gives him monstrously superhuman strength, seen in ANH’s introduction.
He is overwhelming and unrelenting. You could damage his suit, cut off a limb, destroy his respirator and he’ll just keep going (the Obi-Wan show didn’t understand this). ROTJ is an exception since he let go of the dark side.
Alongside Episodes 5&6, SC 38 I feel like perfectly showcases Vader’s fighting style, as does his fight with Cere in Jedi Survivor.