I'm not sure what the correct term would be though. "Natural born citizen" normally means someone who is a citizen through birthright, usually, but not always, because they were born within the US.
The phrase that I've most commonly heard is "Nth generation American", but does this require all members of N generations to be citizens? My daughter is eligible to be a "Daughter of the Revolution", but I'm not (yet) a citizen
Obviously there can be reasons in specific cases, but there should be no reason why it takes longer in general. At least, no reason that I can see. What are you thinking of?
It is no doubt overly simplistic to assume conscious racism, but as this thread demonstrates, Americans who are not KKK racists nonetheless have a lot of subconscious attitudes regarding race that can have an influence.