To me, Speed Metal (allmusic, wikipedia) represents the earliest phase in which Metal really went underground and began developing as its own thing.  I know other bands were there first, and playing Metal (Speed or otherwise) before some of these, but I think these tracks are good representatives of the subgenre.  I also realize that the line between Speed and Thrash Metal is razor thin, particularly since many of these bands fit into both categories (also, plenty of metalheads don't really even distinguish between Speed and Thrash).  Speed Metal is aggressive, with a pounding, rolling beat and driving, often virtuosic guitars.  Don't underestimate the musicianship involved here; these guys play loud, hard, and, above all, fast, without sacrificing clarity or intensity.