Loading times/responsiveness are certainly a design constraint, and designers need to be aware of this. Imagine a print designer that didn't know about bleed,

Loading times/responsiveness are certainly a design constraint, and designers need to be aware of this. Imagine a print designer that didn't know about bleed, DPI or colour showing through paper. They may not personally be in charge of printing things, but they absolutely need to have domain knowledge or work with someone that does.