Limit or disable wordpress heartbeat to help speed up admin screens. Please in functions.php file.

heartbeat-limit

// Limit or restrict WordPress heartbeat to speed up wp-admin when many browser tabs are open or traffic is high
// Taken from http://wordpress.stackexchange.com/questions/166727/how-can-i-speed-up-my-wp-admin-section

function optimize_heartbeat_settings( $settings ) {
    $settings['autostart'] = false;
    $settings['interval'] = 60;
    return $settings;
}
add_filter( 'heartbeat_settings', 'optimize_heartbeat_settings' );

function disable_heartbeat_unless_post_edit_screen() {
    global $pagenow;
    if ( $pagenow != 'post.php' && $pagenow != 'post-new.php' )
        wp_deregister_script('heartbeat');
}
add_action( 'init', 'disable_heartbeat_unless_post_edit_screen', 1 );