The site is loading just fine on my end, about 6 seconds. But yes, I can confirm the builder is loading with the time of 15mins for first 5 tries. Then on 6th up to 10th try, it loads almost instantly. Then now, it doesn't load at all.
Are you on shared hosting? It could be the processing power of your host environment. Even just visiting the admin dashboard could take 10mins in some cases. Perhaps your host could provide some insight as what part is using much resources? I would recommend testing it on another host too, I'm not saying it's really a host issue but just for comparison.