I have problems using
EditBoxes containing large amounts of text.
For example 3,500 lines of text comprising 270,000 characters.
Calling :SetText() and :Insert() with very long strings;
or scrolling, resizing and typing in the editbox can freeze up the game.
It's not really a Legion bug, since it's the same on Live and has always been like that as far as I can remember.
Are there maybe any previous discussions about this?
WeakAuras also has problems dealing with it
http://eu.battle.net/wow/en/forum/topic/17611871083
Edit: I guess the EditBox was not meant for big amounts of text then. If I just wanted to see an in-game plain text list of the amount of character GUIDs I met on the Beta it's already too much to show.