Fast websites don't happen by luck. They happen because someone set a performance budget and held the line — a hard limit on how heavy a page is allowed to be.
A budget forces choices
When every image, font and script has to fit a weight limit, bloat gets cut early. The discipline is what keeps a site fast as it grows.
Ask for it in the brief
If speed matters to you — and for SEO and conversion it does — make a performance budget part of your web design brief. A good studio will already work this way.