Would they have done even better without the racism? Or what accounts for the difference?
That isn't to say that other ethnic groups don't deal with racism too. But it's different. An African American executive in a suit may be mistaken for being a waiter, but he's unlikely to be asked where he's "really from" like an Asian American.
How can you say this with complete certainty? How could you ever be sure?
I think it's easy to point at something like slavery as the underlying cause, but Europeans had no issue looking down on blacks before they had the idea to import them to the new world as slaves, and Europe adopted similar attitudes toward blacks with no slaves present.