Death by a thousand cuts. Everyone wants to add just one little resource, until you end up with 8MB pages with 20 seconds time to interactive. They can all make a plausible argument for how their one little resource will improve the bottom line, even considering the performance impact. If you have one person or one team responsible for performance, they're constantly fighting on all fronts just to stand still. You need a business-wide performance-oriented culture, which is very difficult to develop and maintain.
"You can add your just one little resource, but unfortunately the loading time goes over the limit - you need to identify other resource(s) that can be removed first."
Hard performance targets can work, but only if you have full buy-in from management. If you don't, someone is going to persuade a higher-up to make an exception just this one time and the whole thing falls apart.
perhaps I've been unfortunate but everyone I've met who markets themselves as UX are people whose job seems to be to say how will the customer understand this design, how is the customer workflow, and do not give a damn about performance at the level needed to achieve the improvements being discussed here.
