Green’s Dictionary of Slang

(mainly juv.) wonderful, first-rate, and as a positive intensifier.

1909		J.W. Carr in ‘Word-List From Northwest Arkansas’ in DN III:v 406: whizzing, adj. Extremely, unusually. ‘The matron gave him a whizzing big dose of salts.’.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 181: Other superlatives in favour were: [...] whizzing, whiz-bang, whizzo.