Loading slow on checkout page

All pages on the website are loading under 500 ms, but when loading SureCart pages it seems that every time all payment methods are loading from another domain. I don’t know how but it feels like payment methods are inside an iframe or something else, and that frame is loading slow. It takes a few seconds to load. When the payment methods are finally visible, the visitors are gone. Here is a screenshot what all visitors see a few (3 to 10) seconds:

PS. When contacting SureCart support I don’t get a response. Also after whitelisting the SureCart support mail address I don’t get responses. I don’t know why this is happening but I hope you can fix this issue.