A post over on Boing Boing ("The last mystery of the blues: were Robert Johnson's recordings sped up?") drove me right to my sound editor to see how Robert Johnson's performances may have sounded different from his recordings. Here's the result:
The audio is a 30 second clip of "Sweet Home Chicago" played once at normal speed (i.e., the recorded speed) and then a second time about 20% slower (2 semitones lower). (Note: I didn't labor over the crossfade between the clips.)
Pretty interesting, if you ask me.