A legitimate problem we're solving by…
inventing a new templating language.
Did we need a new one to do this? Absolutely none of the hundreds of existing languages could have possibly worked, so let's reinvent SSI with different syntax?
They'll be infinitesimal faster for the server which will sit idle most of the time anyway... and much slower for you when you make a minor header/footer mod locally, and then you need to re-upload all pages that have a header (i.e. all pages).
I'm sorry, but this project feels more like "art" than a practical tool.
Plus, with a few basic lines of PHP (available on every shared host) you can pre-generate your site statically as well (and on the server, to save you the re-uploading).
Presumably the 're-upload all pages' bit can be automated. As far the reset, the trade-off depends on how often you're updating content versus the amount of traffic coming to your site; in many cases, a tiny saving on frequently-occurring page delivery is well worth a big delay on one-off page generation.