How to fix the problem with the low-quality featured image?

Updated on October 13, 2020

Install and active Regenerate-thumbnails plugin: https://wordpress.org/plugins/regenerate-thumbnails/
Navigate to: Settings -> Media and set to same with bellow image (Image 1)
Navigate to: Tools -> Regenerate-thumbnails then click the button: ‘Regenerate Thumbnails for All attachment’.
Wait for all done and reload the Front Site

Thanks