A save button whose label is its loader - the letters thin into bars that tip like dominoes while it works, then blur into a saved state.
A save button that doesn't swap its label for a spinner. The label becomes the spinner.
On click, every letter thins out where it stands and leaves a bar behind. The bars even out into a dim row, and while the work runs they get read one by one, each lighting up and tipping on its base like a domino before standing back up.
When it's done, the bars blur away into "Saved" and the button tightens around it. A moment later it blurs back into the label, ready to go again. It keeps its width the whole time it's working, and only changes size once there's less to say.