Well, if you look up, there's another poster claiming that it takes 2-3 seconds to start a metro app. I also back that figure.
This obviously varies from setup to setup. I mean, my apps (all of them) open instantly. I also played with an HP all-in-one today at Best Buy and the apps there too opened instantly. Not real sure what some of you guys setup is. I'm on an ASRock 8900 GS Extreme 4 mobo (3.2Ghz), AMD Phenom II quad core (not Black) with 12gigs of 1666 RAM, and a 120gb Kingston HyperX 3k SSD and a Radeon 4870.
As you can clearly see I don't have the newest bits except the SSD, and none of them were top tier even when they were new. So... only thing I can think of is Nvidia. People with Nvidia cards seemed to have more issues than others through the various previews.