...and to suggest a starting point for bodragon's question about origins, the Latin warning Festina lente might be involved. (This was well-known, and used, in Victorian England. For example, someone in Iolanthe sings
Recollect yourself I pray
And be careful what you say
As the ancient Romans say: 'Festina lente'
For I really cannot see
How so young a girl can be
The mother of a man of five-and-twenty.
(The mispronunciation is part of the characterization.)
Of course, this just suggests the same question (who thought of the oxymoron?), but in a language pre-dating English. (And Latin may have borrowed it from Sanskrit, and ...)