Thrash metal is a more aggressive version of speed metal and uses much more hardcore punk influence. Also the subgenre is obviously highly influenced by NWOBHM as is speed metal.

good thread. I think the difference is minimal,if any.Its all based on speed for the most part.Of the "Big Four" ALL were faster than Metallica.Anthrax had more melodic vocalists.Slayer's vocalist was more,for lack of a better word,"punk" in his delivery.
Before that,any band considered "precursors to thrash/speed metal" pretty much all sang in a mid to high range and actually kinda sang,y'kno,with melody lines,even Lemmy.
that's what i get out of it anyway.
Oh,didnt the lead singer of Godsmack play drums in Meliah Rage?...I believe he did.I liked them more than Godsmack. I'm not hatin' I just think it's funny