By default, videos embedded in WordPress posts are missing the enablejsapi querystring parameter that allows YouTube’s iframe API to interact with them. Fortunately you can filter the results of embedded media using the oembed_result filter. Here’s an example:

function my_youtube_player_iframe_api( $html ) {
	if ( false !== strpos( $html, 'youtube' ) ) {
		$html = str_replace( '?feature=oembed', '?feature=oembed&enablejsapi=1', $html );
	}
	return $html;
}
add_filter( 'oembed_result', 'my_youtube_player_iframe_api', 10, 1 );

OEmbed results are cached as post meta, so once you’ve modified the output you’ll have to delete previously-cached URLs before the new filter will take effect for existing posts.

Once you’ve done all of that you can configure and use the iframe API to track interactions, manipulate video controls, and do anything else that the API exposes.

Sometimes it’s useful to add the async or defer attributes to your script calls in order to prevent that script from blocking the rest of your page from rendering. This is particularly useful with third party scripts which you do not host in the same place as the rest of your site.

If you need to load an enqueued script asynchronously in WordPress, or modify the <script> element in any other way, you can use code like the following:

/**
 * Add async attributes to enqueued scripts where needed.
 * The ability to filter script tags was added in WordPress 4.1 for this purpose.
 */
function my_async_scripts( $tag, $handle, $src ) {
    // the handles of the enqueued scripts we want to async
    $async_scripts = array( 'some-script', 'another-script' );

    if ( in_array( $handle, $async_scripts ) ) {
        return '<script type="text/javascript" src="' . $src . '" async="async"></script>' . "\n";
    }

    return $tag;
}
add_filter( 'script_loader_tag', 'my_async_scripts', 10, 3 );

The script_loader_tag filter was added in WordPress 4.1 for specifically this purpose. It is run whenever a script tag is generated by WordPress using the wp_enqueue_script() function. In this example we compare the script $handle against a list of known scripts that we want to load asynchronously.