This is something really obscure that I noticed; throughout the Saga, the lightsabers ignite at different speeds! The best example is in Empire, when Luke's blade shoots out quickly, but Vader's ignites slowly: This has been done since A New Hope: Anakin's lightsaber ignited quickly, while Ben's was slow. This a really minor detail, but it had to be done with intention. I think behind the scenes, it's simply a style choice (more of a necessity in the PT). Not sure if there is an in-universe explanation.