When I did my intensive PPL in the US, I studied the theory beforehand (took me more than two months, 2-3 hours in the evenings, 2-3 evenings per week) but I passed all but one of the exams in the first week there, on the first attempt.
The only exception was nav, which I deliberately postponed until after I had done a navex or two. I passed that one on the first attempt too, but only at the end of the second week.
It's not the exams themselves. They're only 90 minutes or so, so they can easily and comfortably squeezed in between your flying sorties. (Due to instructor scheduling, there will be at least two hours, and more often four hours between flying sorties.) What is going to kill you is the study for the theory. That is something you definitely should not postpone until getting to the US. Ideally, you should be ready to take *any* exam on the first day of arriving in the US.