Don't try to load huge file into Javascript WYSIWYG editor. The Javascript is run in interpreted mode, and once the whole text is loaded, the parsing and formatting begin. A better solution would be to break down the text into sections, displaying summary and put edit button by those sections.
var sHtml = '<img src="' + sUrl + '" alt="' + piDesc + '" title="' + piDesc + '"/>';
tinyMCE.activeEditor.execCommand('mceInsertContent', false, sHtml);
TinyMCE - Adding custom toolbar icons to a plugin