

The only way to keep his body moving the way he needed to was to fill his body with dark side energies, similar to how Yoda was able to flip around at his old age, but Palpatine grew tired of the effort that required, just relying on easier techniques such as the hate fueled sith lightening. Palpatine, being darker than ink on a solar energy panel trapped in the depths of the ocean, rapidly lost his ability to fight hand to hand with lightsabers. By the time of Episode IV the dark side has taken a heavy toll on his body and he is no longer the swordsman he once was, Obi-Wan was just old. The only reason he didn't defeat Obi-Wan on Mustafar was because he was too confident in his newfound power. Thus when Vader first turns to the dark side his power is vastly increased, giving him the ability to carve through the Jedi Temple.

While the Dark side provides a vast increase in power early on, one's body is not built to handle the negative energies flowing through you, I even hypothesize the force itself tries to destroy your body as revenge for abusing the force. While canon has pretty much abandoned said idea, I still cling on to it for it is my last hope haha. Well I offer an explanation I am pretty sure has been provided before but I figured I might as well bring it up again. Beyond the practical explanation of why the lightsaber battles of the PT were more thrilling than the ones in the OT (search your feeling, you know it to be true) us superfans (i.e., nerds) have tried to explain why the skills of lightsaber combat declined so rapidly.
