You think he surpasses Kenobi in Sabers? Wow. The guy was having a difficult time against Old Ben.As for your "gained speed but lost strength" theory, it doesn't matter because Lucas makes it kind of clear in the ANH commentary that Vader and Old Ben were not what they used to be in terms of blade work.
Lucas kind of contradicted that idea by introducing Dooku who is much older than Kenobi and Yoda who can't even walk properly - both are example of prime Jedi.
Yes, Vader lost mobility. But it would matter only on uneven ground like for example Mustafar or Felucia. Yes, old Ben is out of shape, which means his endurance is decreased but skill should still be more or less the same. One can't simply forget decades of training. While it can be the case for normal human, Force users use the Force which allows them to remember things like it was yesterday. Imho both can get tired faster than in their prime but still demonstrate top level performance the same way Dooku and Yoda did in short duration of time.