Someone needs to give the marketing team a 'budget' just like how they have a financial budget they must stay within.

300ms average load time added by the marketing team, or 200kb JavaScript, or whatever. Split it all up however you like, and let them fight amongst themselves for what goes live on the site.

That won't work for the developers who lack the authority to make this a rule, but that's what you go to management with along with any of the various studies showing how important speed is to users.