League of Legends (LoL) Question: Steel Legion Lux slower autos?

I noticed this too when I purchased the skin. I practiced farming with the skin in custom games because as you said the auto-attack animation is different from default. If you don't want to practice the animation and you feel like it is affecting how well you farm, then you should either buy a different skin or use the classic one. You shouldn't use a skin that negatively affects your play.