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If we go back to the 1990's to put the question in better perspective, I would say that for the most part when foreign female performers went to work on US soil, they invaraibly gave inferior (sexual) performances than in euro productions. I cannot speak for the actual or perceived reasons for that, but I doubt it had to do with US filmmaking methods in wanting to keep things more "subdued".
To name a few: Channone, Deborah Wells, Tabatha Cash, Miss Pomodoro. The sex performance(s) of Miss Pomodoro in US productions were close to being lifeless and catatonic. I don't think Moana Pozzi put any more sexual heat in US movies than she did in her native country. Although she only shot one movie in the USA, Lili Carati's performance in that one was just as dull as the handful of her Italian movies. I think for the most part they were just there to collect a check and perhaps to (try to) bolster their porrn career.
You may want to suggest that there is language barrier that may have made things dificult for these performers to really give it their all, and that they were pioneering foreign porn actresses heading in untested waters, but you have 18-19 year olds from europe in recent years who shoot in the USA who can barely say "hello" and who are hot as hell. So language could not have been a major issue. We have to conclude they did not care much, and just went through the motions.

To be a bit cynical [or realistic?] the reasons they're there are, to me, largely
immaterial. All I require is that they put on a moderately convincing performance
[which, after all, is only what many of us are expected to do in our 'normal'
working lives].

I agree with Len that many Euro gals are better in their native idiom, as it were;
and am equally at a loss as to the reason/s.

JJ said: I agree with Len that many Euro gals are better in their native idiom, as it were; and am equally at a loss as to the reason/s.
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I qualified my comments in my previous posts in setting them in the 1990's (since we were discussing Channone's euro and US performances).
In that respect it would be fair to say that the Euro gals WERE better in their their native habitat.
Of the handful of Euro porn actresses that lived and worked in the USA in the early 1990's either for short or long periods (or those that went back in forth a few times) I would say that generally their sexual performances were a tad inferior than in their european movies.
By the same token, and to be fair, let us briefly mention some US porn actresses that went to work in France/Europe and what they did there in the 1990's: Tracey Adams, Sunny McKay, Madison, Carolyn Monroe, Jeannine Pepper. Tracey Adams did not turn out any sex scenes there that were any hotter than what she did in the US (perhaps "glosser" work if you consdier the VMD movies). Same could be said for Sunny McKay, except for one rare golden shower scene (partially faked in close shot). Madison only did a couple of movies in France and they were dull. Jeannine Pepper did some golden shower scenes and perhaps she went a little farther than what she could expect do in US movies. Carolyn Monroe seems to be the exception to the rule, in that she acquitted herself much better than in her US productions (with that lame boyfriend of hers). Perhaps all in all, the US actresses mentioned above (I cannot recall any others that had a considerable euro output) kept it at the same level if not better.