> It would literally be many thousands of times slower(or worse) than doing it locally where the templates can be in memory as you render the pre-render the pages.

Yeah, it's not. I'm doing client side includes and the includes get cached by the browser. I'm sure I would have noticed if my pages went from 1s to display to 1000s to display.

If you have a site/webapp with (say) twenty pages, that's only two extra requests for both header and footer.