Video shows Nokia Lumia 620 speedier than Nokia Lumia 920 in some functions
boot screen image up and running. When booting up to a Wi-Fi connection, the Nokia Lumia 620 beats out the Nokia Lumia 920 and occasionally, the lower end model loaded an app faster. In those cases when the high-end model was first, it wasn't by that much.
Perhaps some of the reason for this has to do with the newer Adreno 305 GPU that is on the Nokia Lumia 620 compared with the older Adreno 225 on the Nokia Lumia 920 (although the Nokia Lumia 920T made specifically for China Mobile sports the faster GPU). Those with low to mid range Lumia models might not be missing out performance wise compared to those who shelled out more money for the Nokia Lumia 920.
source: MyNokiaBlog via WMPoweruser