People believe a slow site is optimised at the end. It is not: the expensive decisions are taken at the start, when a three-hundred-kilobyte animation library is chosen for an effect that needed ten, or when four remote fonts are loaded for three heading levels. No later optimisation makes up for those choices, they can only be undone. So we settle them at the mock-up stage.
The second principle is that a site must work without depending on anyone. Every third-party resource is a point of failure, a data leak and an admission: a font loaded from an American server transmits each of your visitors' addresses before any consent, which makes your own privacy page untrue. We bring everything in house, fonts included, which costs a little space and settles the matter for good.
The third is that accessibility is not a moral option but a manufacturing constraint. Sufficient contrast, keyboard navigation, coherent reading order, alternative text that describes rather than names. It benefits the people concerned first, and incidentally everyone else: the same rules make a site usable in sunlight, on an old phone, or one-handed on the underground.