For me, the plain textarea is visible for a short time (a fraction of a second), but after the page has completely loaded, it is replaced by the editor. Have you tried using Chromium's developer tools to figure out what is causing the lag?

First of all, you'd probably fire up the console (Shift-Ctrl-J) and reload the page to check whether anything is logged to the console. If any errors occur, go to the  "Sources" tab, click  "Pause on exceptions" and reload, this should drop you right into the line that causes the error. If no errors occured, check the "Network" tab to see if anything takes exordinary long to load. If that's not the case, you could also check the "Timeline" tab (short introduction here), anything causing a 20 seconds lag should be clearly visible there. This should give us a better idea of what actually causes the delay.

Btw, is there any extension you are using on all your browsers and machines? Maybe a rouge extension might be causing this (just an idea, I've recently had had a bad experience with an extension I had installed).

Holy shit, the html document is almost 54k lines long! I guess that's where your lag comes from -- for me it's only about 400 lines. My best guess is that these two admin bars you've got there are bloating the document, even though I've got troubles believing that they could possibly bloat it so much.

I know what issue is causing this loading lag, and it only affects administrators.  It has to do with Drupal needlessly rendering a massive amount of information about template variables.  I tried to remove it, but that ended up making a big mess, so I put it back.  I'm sorry for the trouble. :(