I don't notice any vast improvement in Vader's swordfighting between ANH and ESB. When the duel starts Luke is swinging all over and pretty active while Vader just stands there and fights essentially by only moving his elbows. Now that can be seen from two sides: he's a slow old guy or he's just so damn good that Luke ain't shit to him. Turns out he's just a slow old guy cuz Luke hits him in the shoulder.

One of the novels says that his suit is so damn heavy and awkward that he has to use the force just to move. This was intentional on the Emperor's part. The bad suit slows Vader down and makes him less likely to overpower Palpatine, plus he's never able to use the force to his full potential because he's always got some of it (and his attention) focused on the suit. Otherwise he'd just kill the Emperor and take over like it was nothing. He actually needs Luke's help to do it because the suit's so unwieldy.

Truth is Prowse could barely move in the thing, so fancy swordsmanship wasn't really possible.

Hang on...isn't it someone else in the suit for parts of the Empire fight? Maybe out on the catwalk where he actually moves around