[BashiBazouk]

20 years? Try going back 400. 99.1% of the US population are descended from immigrants. Ignoring genetic differences and only going with cultural, you are really talking about how quickly immigrant groups homogenize to a standard culture of the country (or region when talking about America). The answer is it varies. A lot. Take any major Scandinavian city. How big and old is the china town? Japan town? Little Italy? German town? Irish population? Puerto Rician population? All the many flavors of Hispanic populations? Asian populations? Slavic populations? African populations? Many American cities have long standing examples of most of these. China town in San Francisco, for example, started shortly after the gold rush. There are many Chinese there who are multi-generational Americans but have not homogenized to the standard culture (if such a thing exists in SF). I have known families in the central coast just south of the silicon valley who can trace their Spanish descendents to pre-gold rush and even have some Ohlone descendents in their past, the people who originally inhabited this part of California. They are very Hispanic culturally.

I read an article a while ago about problems teaching immigrant kids English in the Alum Rock school district. This is just a single school district out of many in San Jose. If you are unfamiliar with San Jose, think south Silicon Valley. They typically have to deal with students who speak 40 different languages. Now that is diversity. Anything similar in Scandinavia?

As I have never been to the Nordic countries, I think it's possible but highly unlikely they are anywhere close to the diversity of the US. I would need to see a lot more than 20 years of immigration statistics to believe it though...