Checkout/cart extremely slow

I experiencing some issues with the custom products created with fancy. On our website (using Avada theme and woocommerce) we have both regular woocommerce products and customizable products created with Fancy. The Fancy interface works fine. The checkout process of the regular woocommerce products is perfectly normal. It takes a few seconds. While the checkout process with even just one single Fancy product takes 2 minutes and 15 seconds. Both from pc and from mobile. During checkout, once the last confirmation button is clicked, the page just stays like that for a very long time. We receive the order in the backend and on the frontend the page is still stuck. It's extremely confusing for the user, especially because it happens coincidentally with the payment stage. 

Is anyone else experiencing this? Any tips on how to fix it?