What about data we have from many countries today? PISA tests!

Why do people of X-extraction who were born in Y-country score about as well (or typically badly) as people back home in X-country? They do this in Western countries with sufficient nutrition and free education. They do this even when their parents immigrated as children. They even do this when their grandparents were the immigrants.

And the sample sizes are huge...

PISA tests are not IQ tests.

Cultural and socio-economic factors are just as good an explanation for the effects you mention than genetic ones tied to race.

I also very much doubt that the effects you mention are studied in the data on PISA test takers - there is simply not that level of demographic information being collected when a PISA test is administered. No one asks each child in the classroom where their parents or grand-parents were born, nor is their race or skin color noted down. So until you cite some papers, I'm going to be extremely skeptical of your claim.