When did the WASP population peak in NYC, Chicago, Boston and Philadelphia?

[King of Kensington]

I'm guessing some time in the early 20th century (though the date varied by city - perhaps it may have been earlier in Boston which had smaller city boundaries for example).  In the 1890 census "native whites of native parentage" probably would have been good in terms of capturing the size of the colonial descended population*, and add to that British and some Canadian immigrants.  By the 20th century though it would be more difficult as there are now lots of American-born to American parents descended from the 19th century.  Hence by say 1920 there are lots of Irish ancestry in Boston and many of Irish and German ancestry in the other three cities in the NWNP population - perhaps even outnumbering the "old stock."

* Would include those the Dutch in NY and the Pennsylvania Germans in the "WASP" category.

[King of Kensington]

In NYC, perhaps sometime after 1900?  Westchester's population surged from 184,000 in 1900 to 521,000 in 1930.  Not all of that would have been WASP elites though; Yonkers/Mount Vernon would have taken in a lot of that growth.

[King of Kensington]

The percentage of the population that was WASP obviously dropped through the 19th century in all these cities, but my question is when they hit their population peak.