This isn't strictly true.
Most B2B SaaS's, which I would bet is the most common front end developer on HN, do not see significant benefits from fast loading. They will see issues with slow loading, and they can't ignore bundle size, but the benefits to development speed from a pure JS front-end from this kinds of sites is realised as value by the client.
B2C SaaS, and you start to see a different story, largely due to the wider variety of devices and more mobile friendly focus.
Non SaaS, product sales or news focused businesses facing joe public are likely as you say.