Green’s Dictionary of Slang

a general term of admiration.

1910		‘O. Henry’ ‘The Fifth Wheel’ in Strictly Business (1915) 66: I saw the tire drop off your whiz wagon.
1942		W. Winchell On Broadway 12 Apr. [synd. col.] John Garfield in a whiz performance.
1958		M. Allingham Hide My Eyes (1960) 62: ‘Hullo, darling, how were the shops?’ ‘Absolute whizz.’.
1966		I. Vaughan These Were My Yesterdays 39: It was a lovely dance with a whizz supper.