Robert Johnson on speed....

Musicologists are convinced blues icon Robert Johnson's recordings as released are 20% faster than he performed in two solo sessions in 1936 and 1937. It's unclear whether they were sped up intentionally (to push their excitement, which seems hardly necessary) or accidentally at some point in the chain between microphone and pressing plant.

What is obvious is that since only 11 of the 41 existent Johnson takes were issued by Vocalion on 78 rpm discs during his lifetime (and one posthumously), his complete documented repertoire of 29 tunes issued on two Columbia Records lps, King of the Delta Blues Singers (1961) and King of the Delta Blues Singers, Vol. 2 (1970) and finally 41 tracks, alternates and all, released on a best-selling 2-CD boxed set, Robert Johnson The Complete Recordings by Columbia in 1990, we have probably never heard what the blues' most influential singer-guitarist actually sounded like.

Pity the generations of guitarists trying to match the speed and dexterity of Johnson's recordings, as they had them at the dawn of the British blues scene (Eric Clapton, Keith Richards, John Mayall, Jimmy Page and Jeff Beck are all Johnson acolytes) and the emergence of the white American blues revival (ditto Mike Bloomfield, Jerry Garcia, Duane Allman, Stevie Ray Vaughan, Jimi Hendrix, John Hammond Jr., etc.). Of course it's possible that the untoward challenge of aiming for an impossible goal produced faster, smarter guitarists than if they'd been imitating the somewhat more relaxed pace of Johnson as he really played.

But still -- 20%?!? Listen to the examples of Johnson as we've always heard him and as he apparently sounded during his brief, intense life. Muddy Waters, Howlin' Wolf, Robert Jr. Lockwood, Honeyboy Edwards, Johnny Shines -- these bluesmen heard Johnson live, so they probably weren't so effected.
 
The razzle-dazzle clip of the guitar as heard in the electric blues and related genres (psychedelia, prog rock, heavy metal) all these years may sprout from a mastering error as much as from young mens' (mostly -- check out Rory Block, Bonnie Raitt, Janis Joplin) irrepressible energy and eagerness to imitate the hand-jive of a mythic musician whose legacy, it turns out, is somewhat slower -- more deliberate, more aching, even more emotional -- than we've always thought.
http://www.artsjournal.com/jazzbeyondjazz/2010/06/robert_johnson_on_speed.html