In the 1922 Wimbledon final Suzanne Lenglen defeated Molla Mallory, 6–2, 6–0, in 23 minutes. Some accounts state that the match was over in 20 minutes.
In the 1925 Wimbledon final Lenglen defeated Joan Fry in 25 minutes, 6–2, 6–0.
But it was not called WTA during this period !
But what Steffi did in a Slam final is really impressive.
Did she even do anything?
The belief that man is an irresolute creature pulled this way and that by two forces of equal strength, alternately winning and losing the battle for his soul; the conviction that human life is nothing more than an uncertain struggle between heaven and hell; the faith in two opposed entities, Satan and Christ - all this was bound to engender those internal discords in which the mind, excited by the incessant fighting, stimulated as it were by the constant promises and threats, ends up by giving in and prostitutes itself to whichever of the two combatants has been more obstinate in its pursuit. Life isn't black and white, it's gold.
Kim's demolition of Safina at the AO couldn't have been more than 45 minutes. I also remember Serena bitchslapping Capriati in 45 minutes at Wimbledon one year. But yeah, the '88 FO final is by the shortest I can recall (before Li's match today, that is).
Serena's shortest (to my knowledge) is her 35 minute demolition of Basting at Wimbledon in 2000. At that same tournament she also beat Raymond in 41 minutes and Tanasugarn in 46 minutes, in the QF and R16 respectively.