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1. Blocks incremental rendering?
Indeed ... for people not TeeWEE, "incremental rendering" means say a 2MB HTML file will be partially rendered prior to the entire payload being transferred. That is no longer the case here. 100% true. As an aside, I'll think hard about how to overcome this.
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2. The description of how it works?
Correct. Very succinct. Good Job!
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3. Significantly slow down a page?
This depends on the page. If it's a small-footprint HTML page which leverages templating, then the base page will be quite small.
If the design previously loaded unneeded dependencies which were only applicable to a subset of users, then this provides a very easy way out without requiring lots of technical knowledge - and probably a good speedup to boot.
As far as actual tests, I've refactored a number of personal projects with this library, and the time to where the spinner stops (if I load the two versions side by side) isn't noticeable through my eyes. I can't honestly tell which one is which.
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4. A "mis-use of JS"?
It's certainly non-canonical - but I'd argue that many "standard" common practices got their start being just as heretical and then were later replaced by standardized ceremonial ways of doing things.
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I can't tell you how many hours of time I've wasted on non standard "Standard" things simply because it saved some other guy 20 minutes in time somewhere else. It's a nice hack, but let's leave it at that.... try to improve your resume with other methods.
The way TopLevel works, is by getting the complete HTML content, putting it trough a javascript parser, and then rewriting it using document.write.
So this requires all content to be downloaded first, and the toplevel.js to be loaded, before a page can be displayed properly. This breaks all the improvements made in browser, and would significantly slowdown page use.
Also, its completely non standard, and a mis-use of js.
Nice hack however ;-)