Your wiki index text is 100,644 bytes long. The default size limit for a wiki page is 100,000 characters, which can be changed in the Settings (in UTF-8 bytes:characters is not usually 1:1). However, I do not recommend making the limit higher. You can tell that there is already a performance drop on the wiki index due to approaching (in this case, surpassing the limit). In our tests, pages over 300,000 characters long will never load under XenForo before PHP times out (or the CGI times out, where PHP has no time limit).
By comparison, vBulletin will accept close to 500,000 characters before timing out. Surprisingly, vBulletin's parser seems to be more efficient for long-length pages. This is not to say that it is better; in fact, XenForo's parser supports more customization and is more logical in its design.
There is a further performance hit because the datatables BB-Code instantiates a second parser inside the main one that runs recursively inside table codes. I already mentioned to the author that this approach is both unnecessary for that add-on's purposes and error-prone, but I think they didn't want to refactor their code.
Even though you haven't actually tried to save the text, XenForo checks that the limit is not exceeded when you change editor modes too. If you don't like this behavior and think the error message should only show if you are saving your content, you should take it up with the XenForo devs. However, I suspect they implemented the limit on the editor-switch to prevent excessive processing wherever message data is handled.
I would strongly advise you to simplify/shorten your index text and possibly move more detailed information onto separate pages.