I don't know what you think but I am one of those ballbreaking metalheads that are used to consider the so-called speed metal and thrash metal as two different genres. Despite this, there are no real differences between them for many other persons with the result that they uses the term "speed metal" as synonim of the thrash metal. But the question was more confusing during the mighty '80s when, to mention a notable example, Metallica were described as a speed metal band if not even a power metal band, just because they titled "Power Metal" an obscure demo of their early days thanks to an idea of their bassist Ron McGovney. Therefore, speed metal is considered by many not a real metal genre but a definition born and dead in the '80s.
In general, speed metal is to be considered like a kind of sped up pure heavy metal that's more linear and less violent if compared to thrash metal, that, instead, is a lethal combination between the pure heavy metal and punk/HC, so the style created by bands like Slayer or Metallica is more nervous and ferocius than in the speed metal way. To name two examples so to understand better all these differences, speed metal is represented by "Heavy Metal Maniac" of the Canadians Exciter while thrash by "Kill 'Em All" of Metallica, both albums released in 1983 and both expressions of a very fast metal, just played in two different ways: in fact, the first one is more epic and metallic while the latter one is more scrapping and punkish. And still, Metallica and Slayer (so, the entire thrash metal movement) reached the worldwide reputation (and a bunch of money!) in very few years while Exciter and Impaler (ergo, the speed metal) became cult bands adored by the most metal fundamentalists.
The far origins of both speed and thrash metal were into the '80s, in particular due to the fast "Exciter" (1978) of Judas Priest. This is a song elegant but brutal (granted, for the standards of those times!) to be included also as one of the very first examples of the use of the double kick drum bass in rock music. But, for the truth, the '70s were a good time for single songs able to challenge the listeners in terms of speed and aggressiveness, even by the most unthinkable bands. For example and going beyond the usual suspects Deep Purple ("Speed King", "Fireball", "Hard Lovin' Man"...), comes to my mind numbers as "Frustration" of Jerusalem (their first and last 1972 album was produced by nothing else than Ian Gillan!), the schizophrenic and hyper-aggressive "Plastic Shotgun" of the Germans Night Sun (1972), the proto-motörheadian street metal of "Damned Flame" of the Belgians Blast (1973) or, finally, the very hard "Stone Cold Crazy" even by Queen (1974)!
But all these aforementioned examples were only isolated episodes made by bands that experimented new killing tempos without creating a movement focused on fast songs and neither proposing a systematical codification of the speed/thrash metal, destined to starting from 1983 also if preceded 3 years before thanks especially to "Ace of Spades" of Motörhead.
So, here you are a short story about the proto-speed/thrash metal from 1980 to 1983, and we will focused especially on the bands that believed so much in the speed that this one became (more or less) their belief.