There's always something running in the background (usually dozens of system services) so that's not a problem - plus they scale their consumption depending on power.

Plus the waiting on IO is when they're NOT doing something. When they do (playing video, listing files and rendering thunbnails, processing video in an editing app, rendering the next frame in game, playing a synth part in a music app, applying an effect on a photo, recalculating an Excel after a change, and so on) is when you want them to be fast.