I'm in the UK. On S3 the coms often don't work and you get disconected from the game for no reason. And the lag is worse. None of this happened on S2 even with ADSL.
So what are the technical reasons for S3 being so... well...
Where I'm at (USA, Ohio) the lag is far more worse on Socom 2 than 3 ever has been. Have you tested your internet speed to make sure that's not the problem? Also are you playing in big maps or small? Big maps have a lot of lag in them at times and sometimes the host has a bad internet connection making it lag.
I think that it could just be the size of the maps. Because their oftons of people on one room and one host has to support them all. Also the amount of grenades thrown at one spot in the beggining of a round doesn't help. So many explosions could also cause lag.