It just seems like the x4 620 is slow compared to the Phenom II in terms of clock speed. It's 2.6ghz compared to 3.1 on the Phenom.
This is only a problem in single-threaded apps that thrive on clock speed. For multitasking, four cores win every time. In Tom's review of the 620 they could even game while running a virus scan in the background, with almost no performance hit. Try doing that on a dual-core?