Green’s Dictionary of Slang

(US black) a smart, alert, intelligent man or woman; thus soon adj., smart, intelligent.

1884		J. Harrison ‘Negro English’ in Anglia VII 262: De soones’ nigger on de plantashun = the cleverest, &c.
1900		A.H. Lewis ‘Mulberry Mary’ in Sandburrs 11: She d’ soonest frail that ever walks in d’ Bend.
1922		A. Gonzales Black Border 327: Soon-man – a smart, alert, wide-awake man.
1930		G.B. Johnson ‘Speech of the Negro’ in Botkin Folk-Say 357: Soon, early, quick, alert. ‘A soon breakfast,’ ‘a soon man’.