We have been talking about the laggy menusystem because it is serverside.
What I find very annoying are the error-textboxes (like "You already have that item", "You can't build here", "You don't have money", ...), because it takes very long to close them (because of the previous mentioned system) and you can't do anything until you closed them.
So I suggest just to show a textmessage (just text, like a chatmessage) in order not to disturb the gameplay.