At what point does a static site need a backend

When you need to keep a secret or write shared state. Everything else, including search over your own content, can be done in the browser with a prebuilt index, and it stays working long after any server you would have written stops being maintained.

font-display: swap plus a fallback stack with matched metrics. The modern lever is size-adjust and ascent-override on the fallback @font-face, which lets the substitute occupy the same space, so the swap becomes invisible rather than merely fast.

Because the browser does not know their size until the bytes arrive. Set width and height attributes, or an aspect-ratio in CSS, and the space is reserved from the first layout pass.