What happens is the customer is submitted to a page on Authorize.net, then Authorize.net contacts a page on your server, that page determines if the transaction was successful or not, then a snippet of Javascript is sent back to forward the customer to the receipt page. So if the connection between Authorize.net and the page on your server is slow, or if it takes a while for the customer to load your receipt page, they're going to sit there for a bit staring at a white screen. How is your hosting in general? Does it always respond instantly? If not, then that's one thing you can fix. If it does, then you could try increasing the refresh time on your DNS from the usual 1 hour to something much larger, like 7 days, and see if that helps. But there may stil be a few customers on connections with really lousy nameservers that lag while ordering. The default nameserver for my Southwestern Bell DSL lagged horribly starting back some months ago, I finally figured out that I could switch to the nameserver from a different city, and apparently the one I went with isn't as overloaded, since I haven't had the problem since. But if I were still using that, I'd get 5-10 seconds of lag loading the Authorize.net page, then maybe another 5-10 seconds loading the receipt page if it's on a different domain from the order form.
