I have the same problem. Ironically, Microsoft's own website https://products.office.com refuses to open! When it does open after waiting a long time, it opens in the minimal HTML mode where the menus on the website are rendered like a nested unordered list.
I first noticed this problem about a year ago on my Lumia Icon. Soon after, I installed Windows Phone 10 technical preview on it, where the problem was not seen! That was one of the reasons I was forced to continue using Windows Phone 10 technical preview despite other issues at that time. (Now it is much better, so it's a different story :-)).
This week I bought a new Lumia 640 with Windows Phone 8.1. I was hoping the problem would have been fixed by now. I was wrong.
This is one of those severe issues due to which I don't recommend windows phone 8.1 to others. Some issues can be tolerated, this one is not one of them.