As far as Kenobi being weaker, we should remember that he's a broken man. He even tells Organa, "I'm not the man I used to be." The dude lost almost everything he cared about and has been eeking out a hard scrabble existence in hiding on the ass end of the galaxy for a decade. He hasn't been communing with the Force or training, and when he ignited his saber while facing Vader it was probably for the first time in ten years. Vader, on the other hand, is an absolute beast who has only gotten stronger and dominates through strength and brutality. Did anyone notice that Vader wielded his saber with just one hand? It reminded me of his fight with Luke in ESB, where Vader also wields his saber with one hand as he toys with his opponent, but switches to two hands when he gains some respect for Luke's abilities. Notice that he gains no such respect for Obi-Wan's prowess in this fight. Obi-Wan is weaker than Luke was in ESB.
I don't mind if it looks like things have slowed down a bit. Personally, I think a lot of fighting in movies looks silly these days. Too cartoonish. If Vader can't leap around like he used to, well, he's got three artificial limbs and needs help breathing. And Obi-Wan has slowed down because he's a broken man.