Because i know before rebels alot of people would call him a clunky,clumsy slow cyborg torso light bright. Everytime you mentioned how much of bad dude Vader was interms of speed,lightsaber dueling and force skill, they would immediately hearken back to the old films and use that as proof of a lack of speed and any impressive feats. Even though the books and comics told quite the opposite tale. So i guess since Vader is shown being portrayed in the more fluid and fast pace manner of Star War Rebles, its likely that the illusions of his lack of speed and mobility would start to disappear.

Never though he was slow to begin with, even in the old movies you do see the saber combat being quick, especially in the ESB and ROTJ duels. He's not as slow and stiff as people imply even in the movies.

Yes, Vader is actually a cool guy. He's just misunderstood and not a naturally patient guy. He might've force choked a couple imperial employees. But what else is he to do? He tries to be a understanding and compassionate boss or manager. Yet his employees fail and fail time after time. I'd be pretty pissed too, if the stormtroopers under his command can't hit anything with their blasters.

I don't watch rebels, but I never thought him slow.

I never thought he was slow I just thought he wasn't agile. SW Rebels didn't really do anything to dispel that idea.

The old EU comics changed my opinion of him.

Not really. You could tell by his fights in rebels he still focuses more on power than speed. They did a good job capturing his style.

I hated Vader's portrayal in Rebels.

But from the EU, people knew he was relatively fast. He generated afterimages with his lightsaber, contended with Maul, deflected blaster fire, and more.