Lazy Load iFrames
iFrames embed other websites in your website as an element. Some famous examples are YouTube videos, Google Maps, Twitter posts etc.
By lazy loading iFrames, iFrames below the above fold will be loaded only when they're nearby the viewport.
If you're using any other performance optimization plugin, make sure to disable lazy loading iframes in that plugin to prevent conflicts. Similarly, turn off lazy loading iframes in your theme setting if there are any.