It's user interface is responsive, but it's not a powerful handset. That's the best way of describing them, and to be honest for a lot of first-smartphone buyers, that's more than good enough in my experience.

And the reason is that UI is written in native code (read C++) using internal native APIs while 3rd party apps written in managed code (using different APIs. Of course it takes more time to launch managed apps. This is clearly a disparity between 1st party and 3rd party apps.

still better than android where everything is just slow and laggish on low-end phones.