Please ask your questions in the comments below. Please check your PHP Error Logs before writing simply that it doesn’t work, it would be impossible to tell you why without it.

How to display more results?

Go see your Post list. There, you have a Screen Options at the very top, click on it. You can change the Number of items per page, this value is used by the plugin.
Is it possible to run the scan or the deletion in the background? With a cron job for example?

It would be too dangerous. A bad update, the page builder disabled temporary or another mishap and the WordPress install will be basically destroyed. So it’s better not. Also, having all this run in the background would slow down the install at times we can’t expect, might also crash and so it would need many debugging tools and screen to follow up the progress. WordPress was not really made for this kind of tasks and it’s better to avoid it with such a plugin as Media Cleaner.

I recommend you to use WPEngine or SiteGround. They are both excellent hosting services and work perfectly with all my plugins.