I know this post is old, but I have been experiencing this often, loading the modal after clicking "Add to Cart" and after clicking "Checkout" it lags for at least 20-30 seconds and spins, but I'll receive the purchase email right away, thoughts?
I also noticed that when clicking "Add to Cart" it will pop the intended modal (cart) with transparent background (doesn't do this in ipad). But when clicking "Pay with Card" it stalls and goes full screen, how would I code to allow for the checkout process to be contained in the modal (like your demo)?
Thanks!