I too have experienced this lag in Legion. It causes weird delays from when I press a spell macro to when I actually start casting, and very often there is nothing telling me that my character is actually doing anything at all - meaning nothing in chat log indicating I'm casting, no casting animation, no casting progress bar being displayed. Eventually I will get a chat log message saying I have cast the spell, but by that time I've been pressing my macro again and again because I'm not sure if I'm actually casting or not. These 2 issues are enough to hinder my performance as whm in Legion (which is the job I have used for 99% of all Legion runs I have been a part of). It's also been happening in all the halls.