according to the episode I DVD commentary, Vader was slow because his bionics hindered him. also, obi-wan was slow and out of practice, and Vader never meant to kill Luke.
Vader would win in the end because of his power in the force. According to current rumors, episode III will showcase Vader's true powers.
Yoda would get his ass handed to him. Think about it. The main reason the Vader fights are slow is because we didn't have CGI back then. Vader has enough power over the force to compensate for his mechanical body. He would destroy Yoda, I mean Yoda even knew this, which is why he left. He couldn't defeat either Sidious or Vader.
I wouldn't be surprised if Lucas used CGI to redo that fight. Bot Obi-Wan and Vader deserve more credit. Look at Dooku, he's 80+ and he's doing flips in battle. Vader could as well.