The term has gained popularity in the recent decade as a way for SEC fans and mainly ESPN to discredit the SEC's major conference rival, the Big Ten.
The mythical claim results from the stereotype of Big Ten teams containing large, brutish players that can fare well in run based offenses in the cold northern part of the country while SEC teams in the warm southern part of the country are often pass based. Somehow it is perceived that run based offensive players are somehow slower.
This mythical claim also goes against recent matchups between the two conferences. From 2002-2006, the Big Ten holds a 8-6 advantage in bowl games.
Big_Ten_fan: There's no such thing as SEC Speed. We own you in Bowl Games.
OSU fan: "But the Big 10 used to win a bunch of bowl games back in the 1940's, so that somehow applies to the present. WAAAAH!!!"