How important is the issue of slavery to understanding racial attitudes in the United States?

Extremely. No other immigrant group has been brought here forcibly against their will and kept as slaves for hundreds of years.

Pretty important. Important for the black people themseves and for everyone else.

I think maybe segregation might be even more important in that it is more recent history. There are still many people alive today who lived during segregation and through desegregation.

Slavery? Not as much as one might think, but it was a huge part of the long term oppression and continuing economic hardship endured by the American Black people, which carries the heft of current racial attitudes. But slavery itself has little current importance, other than that is how we got to this position.