Well, I will share some ideas with you regarding this subject.
Speeding up stuff
If you implement the BlogTOC widget in the standard way, the entire code of the widget is placed in a Blogger Page Element, usually somewhere at the top of you blog.
A good solution is, to move the retrieval-code to a page element that is completely at the bottom of your blog page.
I have moved the code to the bottom of the footer, and that has sped up loading a lot.
The code you have to move is this:
Automatic display of TOC