Past progress is not a sure indication of future progress. Phones have made huge leaps in the past decade, but now the leaps they're making are smaller and more incremental. We're hitting a barrier of how much power we can pack into a device that can fit in your pocket.

This article does a fantastic job of explaining whats up: http://sealedabstract.com/rants/why-mobile-web-apps-are-slow...