I had this same problem in Windows 8.1 and was really hoping they'd get it worked out by now.
Any Metro app- windows store, mail, netflix, anything- takes ages to open or do anything that accesses the web. In some cases, Mail takes longer to open than Adobe Premiere Pro does, which is plain ridiculous. I love the way the apps look, but I can't really use them since everything takes twice as long to open as its desktop counterpart.
It's not my computer, because I have a quad-core 4.0GHz processor and 16 GB of RAM.