I have a huge FPS drop whenever I try to lockpick an Imperial chest. I think it might be because of the visual effects on the lock. This drop also makes it very difficult to pick the lock.
Sorry to see that you had no replies. Many posts on the subject have been made since. Apparently the visual effect around Imperial City chests signifies a Magical Ward that will damage the avatar when the player fails to pick the lock. Many players report that the "lag" in time between what the server sends and when they see it or hear it makes picking the locks almost impossible.
--- Shadowshire .......... ESO Plus on PC NA with Windows 7 Pro SP1