Essentially, Nokia improves webpage load performances by utilizing data compression algos on their servers through Xpress - similar to Opera and Amazon Silk. In between, Nokia decrypts HTTPS traffic - Opera and Amazon apparently don't.

So, effective use of Xpress could be limited to articles (The Verge), magazines and mailbox use could be done through IE or through the mail app. The problem is limited on the Lumia phones but it is a bigger deal just on their S40 phones.