I'd guess that the computing power devoted to the mouse in a graphical environment is always small compared to that devoted to the screen, and thus it should be a good example. (if one used a mouse to manipulate an array of characters, as sounds like a good idea for a spreadsheet, the mouse might be relatively expensive, but that's not what Engelbart did in '68)

The mouse and the browser (and magfrump's similar example of AIM) are probably examples of a phenomenon that generalizes your original question, where the bottleneck to deployment was something other than computer power.