Vader is actually pretty decent as a toon considering how long it's been since they reworked him. He still hits (sometimes very) hard and he's got that essential role in a Palpatine squad.
And that last bit there is sort of important. You put him in there by his lonesome, no Sith or Empire allies to boost his speed, only one rebel enemy, of course it's no surprise that you get pounded.
I do agree that Vader could use a little touching up though. But then so could Maul, Savage, Sidious, ++