1.9GHZ VS 1.2GHZ

[kenny43]

Not necesarily, GPUs are pretty different on both phones and SGS2's is better since the Sony one is an adreno 205, and also the app doesn't use dual core but it DOES affect the performance of the app, I suspect it is because the background procesing goes on the second core (I tested this by myself, locking it to 1 core has lower performance than locking it to 2 cores, by some 3-7VPS).

Finally, as Guilherme said, the SGS2 CPU architecture is also better, and thats already a nice plus.