I don't know about Ace, but CodeMirror only loads a small portion of the file text into the dom. I regularly load 40 kloc javascript files in Light Table without any noticeable pauses.
Yes they both display only visible part of the text, which
Actually Ace was the one that first used this technique for rendering. And c9.io opens 80kloc files without a problem, and have much faster searching and scrolling.