there are 2 types of lag, one is caused by your computer
the other one is caused by your internet connection
if you are talking about internet lag. you should consider getting an better one
if you are talking about computer lag, you should study your pc to find out what part of it is lacking in calculation power.
but as stated before, we cant help you unless you give us some more info.
what kind of internet connection do you have
what kind of graphical card do you have
what kind of processor do you have
what kind of memory do you have.
these are pretty much the basics of having a computer that can deal with lag
and you might be joining servers that aren't based in your country...happens alot to most people.... to find out where they are go to steam servers choose earths special forces and go into filter and choose country and so on
Well this is not quite concerned to the first post but related...
In 1.2.1 i've also been getting some computer lag that i didn't in 1.2... However I just guess it became a bit heavier for the pc... :S