Gameshow host: Heres the question I ask of all our contestants. What made you drop out?
Bob: Well a lot of people think it was the 400 acid trips I took yah know.
Gameshow host: Uh huh, but what was it really Bob?
Bob: Well one day I played Black Sabbath at 78 speed, man.
Gameshow host: Black Sa... And then what happened?
Bob: I saw God.
Gameshow host: You saw God. Well that sounds like true enlightenment to me Bob.
-Cheech and Chong
That's the core of the joke:)
Black Sabbath at 78rpm would probably sound like "normal" rock/blues.
They only did about 5 fast songs.