What did thurgood marshall (who would later become a supreme court justice) mean when he said "90 odd years is pretty gradual?"?

In this quote, Thurgood Marshall is talking about the time between the signing of the Emancipation Proclamation (which was supposed to free slaves in the Confederacy) to the Brown vs. Board case which ruled that "separate but equal" is unconstitutional.

The reason he brings up these two events is because it shows how little progress American society has changed in these 90 years. Even though African-Americans were supposed to be free citizens after the Emancipation Proclamation, they were still treated as second class citizens in the US. They were constantly targets of voter discrimination, violence, and prejudice.