This is a little PHP Class that add google analytics to WordPress without lowering the Pagespeed Index. It saves analytics.js on the server and updates it daily via wp-cron.

WP - Google Analytics

<?php

namespace sayhello\Theme;

/**
 * Get Google Analytics.js and saves it locally, inserts tracking code into wp_footer
 * Advantage: No 2h googleanalytics.js cache
 * +1 Google Page Speed Point
 *
 * @since  0.0.1
 *
 */
class GoogleAnalytics {

	/**
	 * can either be a  PropertyID, an option Key or an ACF Field
	 * @var string
	 */
	public static $property_id = '';

	/**
	 * wp_upload_dir()['basedir']
	 * @var string
	 */
	public static $dir = '';

	/**
	 * wp_upload_dir()['baseurl'];
	 * @var string
	 */
	public static $url = '';

	public function __construct() {

		$this->property_id = 'none';
		$this->dir = wp_upload_dir()['basedir'];
		$this->url = wp_upload_dir()['baseurl'];
	}

	/**
	 * This Functions sets the Google Analytics Property ID
	 * @param $id can either be a  PropertyID, an option Key or an ACF Field
	 */
	public function set_property_id( $id ) {
		$this->property_id = $id;
	}

	/**
	 * runs filters an actions for the instance
	 * @return none
	 */
	public function run() {

		add_action( 'init', [ $this, 'check_if_exists' ] );

		if ( ! wp_next_scheduled( 'sheduled_GAProxy' ) ) {
		    wp_schedule_event( ( new \DateTime() ) -> setTime( 1, 0 ) -> getTimestamp(), 'daily', 'sheduled_GAProxy' );
		}
		add_action( 'sheduled_GAProxy', [ $this, 'update_file' ] );
		add_action( 'wp_footer', [ $this, 'set_analytics_code' ] );
	}

	/**
	 * saves the analytics.js file if not already saved
	 * @return none
	 */
	public function check_if_exists() {
		if ( ! file_exists( $this->dir . '/analytics.js' ) ) {
		 	$this->update_file();
		}
	}

	/**
	 * updates the analytics.js file and the check-option
	 * @return none
	 */
	public function update_file() {
		update_option( 'analytics_regenerated', date( 'd-m-Y H:i' ) );
		file_put_contents( $this->dir . '/analytics.js', fopen( 'https://www.google-analytics.com/analytics.js', 'r' ) );
	}

	/**
	 * echoes the analytics code
	 */
	public function set_analytics_code() {

		$id = $this->property_id;
		if ( $this->is_valid_property( $id ) ) {
			$property_id = $id;
		} elseif ( $this->is_valid_property( get_field( $id, 'option' ) ) ) {
			$property_id = get_field( $id, 'option' );
		} elseif ( $this->is_valid_property( get_option( $id ) ) ) {
			$property_id = get_option( $id );
		} else {
			if ( is_user_logged_in() ) {
				?><!-- Google Analytics: invalid Property ID --><?php
			}
			return;
		}

		if ( is_user_logged_in() ) {
			?><!-- Google Analytics: Property ID <?php echo $property_id; ?>, Script regenerated <?php echo get_option( 'analytics_regenerated' ); ?> --><?php
		}
		?><script id="GoogleAnalytics">
		(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
		(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
		m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
		})(window,document,'script','<?php echo $this->url . '/analytics.js'; ?>','ga');

		ga('create', '<?php echo $property_id; ?>', 'auto');
		ga('send', 'pageview');
		</script><?php
	}

	/**
	 * Helpers
	 */

	/**
	 * checks if given string is a vaild google analytics propery ID
	 * @param  string  $id property ID
	 * @return boolean
	 */
	public function is_valid_property( $id ) {
		return preg_match( '/^ua-\d{4,9}-\d{1,4}$/i', strval( $id ) );
	}
}

$prefix_ga = new GoogleAnalytics();
$prefix_ga_property_id = ''; //could be a property ID, a wp-options key, or a ACF field key
$prefix_ga->set_property_id( $prefix_ga_property_id );
$prefix_ga->run();