Mudras are actually affected by lag in the exact same way Benediction, Hallowed Ground, and Holmgang are, and in the same way all AOE is. All of these actions are tied to animations that play client-side, but need time to register serverside, so the animation can desync from what the server decides you're doing.

If you have a poor ping, this is a consistent delay; but when you deal with packet drops (the most common issue) it's random and unpredictable. HG/Bene actually *do* become slower-firing if your ping is higher or you lose packets, because the game client doesn't send the skill effect until it's done playing the animation. (Lots of skills work like this for the sake of both balance and visual flair - for instance, chaos thrust doesn't deal any damage until its animation completes.)