Let's give the assumption that certain racial/ethnic groups perform better on IQ tests as a whole than others.

If that were true, that would just be a fact. Yes. But why that is matters. And a lot of the why is just plain old racism. The groups who don't score well on IQ tests don't do so because they've been held back from all of the progress enjoyed by the rest of the world.

It's that progress that increases IQ scores. It's not that white people and Chinese people are smarter due to genetics or culture or whatever people use to spout. It's because they've been able to leverage the progress of the world to advance themselves. Black people, Puerto Ricans, and Mexicans, especially immigrants have not been allowed the same access to leverage it. You give them the same resources, the same chances, and over time, you'll see the same increases.

In fact, you do. Because those groups constantly get progress's hand-me-downs. Last year's progress. So they lag behind.