Why move at all
The old site was hand-rolled HTML, CSS, and JavaScript. It was fast and I was proud of it. But it had a structural problem: adding a page meant editing a build script, the shared global CSS, the navigation in every file, and the i18n dictionary, all by hand. Every new page made the next one a little more expensive.
That's the quiet trap of a no-framework site. The first ten pages are cheap. The eleventh is where you start paying interest on every shortcut you took. I wasn't fighting a performance problem. I was fighting a maintenance one.