I found this yesterday and thought I'd pass it along in case it would help others. All of our pages that used the standard DNN text editor control took an extra 1-2 seconds to load. I found this when traced the slowdowns to the PageLoad event and, from there, to the text editor.
The text editor, by default, has a list of "Custom Links" and DNN fills it in with the links to pages on your site. Our site, though, has around 500 pages, and it was taking the text editor that long to build the list. Fortunately, that's a setting in the HTML Editor Manager accessible by the host account. When we turned off "Show Website Links", the slow downs went away entirely.
This has really improved the usability for our staff. There were all kinds of things - like the News Articles module, for instance, that we thought just ran really poorly... it turned out it was all that editor. Any page where the editor was used took a long time.
Actually, the improvement has been so dramatic that I would recommend DNN turn it off by default. I would guess that subtlety slows down many sites out there. The speed increase for editing content was tremendous... I always just thought DNN was slow.
Hope this helps,