The website was slow for me today but the payment page did come up ok.
I had to close tabs a few times and go to the homepage to be able to add to cart. I am guessing it is just the amount of hits it gets during that time.
Large scale retailers usually have their e-commerce websites load balanced and fine tuned to maximise their selling opportunity but there are overheads to that.
For smaller shops it is best to work with their website hosting providers to see if they can offer add-on services to alleviate such issues. Again there is usually a cost associated.
Sent from my iPhone using Tapatalk