The reason why it took Tom's jury so long to deliberate was because one man on the jury was fighting for acquittal (not guilty). Turns out, it was a Cunningham. Why is this significant?

It shows the possible beginning of change in attitudes and social realities concerning the relations between blacks and whites in Maycomb.

Explanation:

In the trial of Tom Robinson in To Kill a Mockingbird even though the night before the trial Walter Cunningham had been among those who wanted take justice in their own hands and lynch Tom Robinson, Atticus "had a feeling" that after tangling with Atticus and Scout that night, the Cunninghams left with "considerable respect" for the Finches. Atticus could have stricken the Cunningham kin from the jury, but, knowing that "once you earned their respect (the Cunninghams) were for you tooth and nail", he decided to take a risk. Atticus had reasoned that "there's a faint difference between a man who's going to convict and a man who's a little disturbed in his mind". As it turned out, the Cunningham relative was "the only uncertainty on the whole list", and he did indeed stand up for the truth by holding out in favor of acquittal for Tom Robinson.