wiki says that Poles were immigrating massively starting late 19 century - History of the Poles in the United States - Wikipedia, the free encyclopedia
Some numbers
Year Speakers
1910 943,781
1920 1,077,392
1930 965,899
1940 801,680
1960 581,591
1970 419,006
1980 820,647
1990 723,483
2000 667,414
Foreign-born population only
Judging by the numbers immigration actually slowing down not growing up, although I suspect that Poles simply started moving to the EU more