roundshort, I think it you listen to the stormtrooper back-chattering in Rogue One you'd understand that Vader is an older model whose processor needs to be warm up after a long stint in a Bacta Tank.
It's likely the same reason why Vader's settings weren't right in his TIE fighter in New Hope -- he still didn't have his equilibrium back, which I would argue Vader didn't get back until Bespin in Empire.
To round short's point and speaking of New Hope, my big problem with Imperial technology is how does a ship get a name like 'Star Destroyer' when it can't even blow up a tiny blockade runner?
Seriously, as to Rogue One, I've never been a big Star Wars fan -- but I have always been a fan of Star Wars production art, going back to the original 'Star Wars Sketchbook' that came out shortly after New Hope. My copy is battered and dog-eared but it is one of my most prized possessions from my childhood. Without even really cracking a copy, I bought my son 'The Art of Rogue One' for Christmas. I have been very impressed with the quality of the art and believe that some part of the success of the film can be attributed to the early artwork to help the identity of the picture. If you get a chance, I recommend it to all.