What exactly to you mean by "marking text" ?... The script editor is lightning fast for me, be it for syntax highlighting or text selection (with either the mouse or the keyboard)...
Since I've no idea of "what" you speak about, I can't know "why"... Please, give precise repro steps.
Totally different code... Mine is optimized v1, theirs is a mix of LL's v2/3 code and their own pre-processor and whatnot...