How to turn off the Google font loading?

The fonts of your slider are loading, even if you don’t use them all, so you could either remove the Google fonts from the Smart Slider's SettingsGlobal fonts / or your slider’s fonts or modify this file, so they wouldn’t load:
Joomla libraries\nextend\fonts\google.php
WordPress wp-content\plugins\nextend\library\fonts\google.php
Magento app\code\community\Nextend\Library\fonts\google.php

In the end there is this code:

<ol><li>return $url;</li></ol>

And you could change it to this:

<ol><li>return null;</li></ol>