None of that shows the rate at which Asian Americans take the SAT, the birth rates of 2.5 million Asian Americans who were here in 1980, or the ages of the Asians who've immigrated since. So saying that we have an extra 17.5 million Asian Americans, and Asian Americans outperform whites on the SAT, therefore 2nd generation Asian Americans outperform whites isn't supported by the evidence presented so far.

However none of that really matters because you ignored my last point.

Other factors such as family income are larger factors that native language.

>Is it somehow easier for Asian immigrants to learn the prestige dialect than Hispanic immigrants?

If it's easier is irrelevant because Asian Americans are more likely to speak the prestige dialect than Hispanics. But to answer your question, yes it is eaiser because Asians have much higher incomes, and they integrate with white communities faster--at least in part because Asian communities are smaller and more spread out.