By and large users are unaware of how much resources something uses, or should use. They don't really care about anything but getting from A to B as fast as possible, with as few interruptions as possible.
Computer resources are just like any other resource; expendable. User will always use more if that means it's more convenient. Human time is very valuable. Accepting multiple dialogues would take even more time than loading a fat page
as an add: there is no single resource you can bind the multiplier to; many sites use no css, but lots of js, or webgl, wasm, tables... There is simply no possible way to foresee what will be slow and what wont