I have performed some tests and reached the conclusion that you should lazy load images, iframes and videos.
I thought I was supposed to leave that job to WordPress but I found I was wrong.
There are several plugins that can help you do that such as PerfMatters, WP Rocket and FlyingPress
Since I like to keep plugins to do exactly what I need, I was trying to find the a very light plugins to optimize images and videos.
The main problems were videos that load a few scripts and resources.
After playing with several plugins, this is the one that won the battle
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LazyLoad Plugin by WP- Rocket
I chose LazyLoad Plugin – Lazy Load Images, Videos, and Iframes over the other options because the plugin does what you want without any complication.
This plugin by WP Rocket adds some unminified code to your pages and loads one script which is 3.23 KB in size.
The plugin also replaces Youtube iframes with a preview thumbnail.
I installed the plugin in all my sites and forget about it
Optimize More! – Images
I kept reading about optimization and code snippets and I ended up reading an article from the thinkweb.dev website.
While I was checking the rest of the articles, I realized that they have a plugin to optimize images.
Optimize More! – Images uses Lozad.js which is lighter than the library used by WP Rocket
I gave the plugin a try and it delivers what it promises.
The script loads only 1.68KB so I fell in love with it.

I ran some tests since this plugin doesn’t have the option to replace the YouTube Iframe with a preview thumbnail but these get lazy loaded in a way that the videos doesn’t load the scripts and resources when the videos are not optimized for speed.
I don’t know if there can be a plugin lighter than this in the future so I can make a long term commitment but I think you should give it a try if you want to keep things as lightweight as possible.
The plugin has less than a hundred installations so I hope it gets the credit it deserves soon.
Run your own Tests
There are other plugins for lazy loading images, iframes and videos but they need more than one script or they have way too many options.
I wanna keep things as lightweight as possible. The less scripts my sites needs, the better.
Optimize More! – Images is the clear winner.
More about Speed
These are some posts where I gather my thoughts about speed optimization tips