Speed Metal is a real genre. Its just heavy metal but faster, or like proto-thrash metal. But in the early 80s everything was developing pretty fast and so everything was called speed metal or power metal(its why metallicas first demo is called power metal, pic related also). Thrash metal wasn't called thrash metal till the mid 80s i think. Also this youtube channel has a great selection of what is speed metal : https://www.youtube.com/@realsteelassault/videos