General Q about developing sites with speed in mind

How much does it really matter? -sprites, repeat background image effects, svg/base64, pseudo selectors that eliminating additional document markup, etc. -fun stuff I explore as I build my responsive site using bootstrap and sublime, with the goal of finding work in the field.

Yet there are so many wordpress blogs, and non-responsive pages full of bells and whistles, it makes me wonder how possibly whether I used one div or three for my little module, or used a sprite or background image matters that much.

The thing is I don’t have a smartphone, -are these non-responsive media heavy sites just a lot slower? In what context does this image and graphic optimization matter - mobile/responsive development? Thank you for your insights.:cool: