Speed is a design decision, not a plugin
Performance work usually arrives at the end, as a plugin, after the design has already committed to everything expensive. That is the wrong order. By then the only moves left are compression and caching, and both are small next to the decisions made in the first week.
The decisions that matter are unglamorous. How many typefaces, and how many weights of each. Whether the first screen needs a video or a still. Whether the thing that animates has to animate before the page can be read.
We give a page a budget the same way an engineer gives a beam a load: a number, agreed up front, that the design has to live inside. It changes the conversation from "can we make this faster afterwards" to "what does this element cost, and is it worth it".
The result usually looks the same to the client and feels different to the visitor, which is the point. Nobody notices a fast site. They notice a slow one, and they notice it as a feeling about the business rather than as a feeling about the website.
Two founders trained in engineering wrote that last paragraph and had to resist putting a graph under it.