The responsiveness of the UI of Windows Phone 7.5 smartphones has never been a real problem, and from this point of view the increase in performance introduced with WP8, although present, is not so obvious. The situation is different with regard to the speed of launching applications, as proven by the video which is shown below.The new Windows Phone 8 smartphones renew the previous hardware platform introducing for the first time in the Windows Phone a dual-core SoC associated with 1 GB of RAM. The operating system itself has undergone considerable change since the previous WP7.5 release.
In the video are compared the Nokia Lumia 920 and the Nokia Lumia 800, based on the platform Qualcomm S2 with 512 MB of RAM and WP 7.5. The differences in terms of speed of launching applications are evident in the below video.
Everything looks much more ready and responsive with Windows Phone 8. As mentioned, a speed that is appreciated especially during the start of each application (the speed of the UI has always been one of the strengths of the previous generation WP 7.5 Lumia smartphones).