When the mouse cursor changes its graphic (hand —> sword —> magnifying glass etc) the game itself stutters for a split second. When fighting multiple mobs and having my mouse go across the screen it does each little stutter individually. When the mouse is not changing its graphic (on mouse over) everything it fine. No stutter when the cursor is just sitting on the target or item. No add-ons installed, repaired, updated, and even changed hard drives.
So far it happens mostly in the newer zones (Nazjatar, Mechagon, any corrupted assaults) but also in towns as well.