A Star War Theory Explains Darth Vader's Improved Lightsaber Skills in Rogue One 

www.cbr.com -BY ROBERT VAUX -Vader's skills in Rogue One are far more terrifying than those in A New Hope. There's a very simple in-world explanation for the discrepancy.
Keywords: star war, theory, explains, darth vader, improved, lightsaber skills, rogue one, star wars