The original version of 20th Century Boy by T.Rex was released in March 1973. It lasted 3 mins and 39 seconds.
There have been quite a number of cover versions over the year, but I would reckon this one from 1979 is probably the fastest take on things:-
mp3: Siouxsie & The Banshees – 20th Century Boy
The b-side of The Staircase (Mystery), the band’s second single, which got to #24 in the charts. Fast, frantic and fun…..and very much enlivened by Siousxie‘s big miaow before the guitars, bass and drums kick in.
THE NEW VINYL VILLAIN 
This is one of my favourite b-sides, and indeed favourite covers. Alongside their versions of Helter Skelter and Dear Prudence.
Talking of early punk single b-side Beatles cover versions under 2 mins, going back to The Damned’s 1m 43s version of “Help” as featured in this blog previously in the “cracking debut single” series. Not as strong a track as Siouxsie’s cover but I’m liking focusing on The Damned and Banshees.