An empathetic pitch
Unlimited data and processing power is a privilege.
Ignoring this privilege adversely affects: user experience because true performance is the foundation of perceived performance; accessibility because every requirement for a baseline experience is exclusionary; the economy as long as our users are paying for each web request with their labor; and the environment because every web request has material costs.
It’s easy to forget that the web is for all people. So it’s a moral imperative to prioritize performance when we develop for the web.