Set the right project properties, *temporarily* set preview window to Best/FULL, and then click the little icon on the preview window's toolbar to save as PNG. Then go back to Preview/Auto mode.
Best/full is super-slow to playback. It's only to be used for quick views of how a color-graded scene or otherwise fully manipulated shot will look like in its final, hence that's why we use that for screenshots. Preview/auto is where the bulk of editing should happen. And if your PC is too slow, use draft/auto.