They are not THAT closely related one is very fast and melodic sounding with clean vocals .Thrash is very brash sounding and has that "thrashy" sound it got from combining the speed, harshness and aggression of hard core punk with the more technical aspects of metal.
Personally, I'm not here to get anyone to "believe" anything so whatever it's all good
The reason I wrote the labels was so you could hear the differences plain as day They are pretty obvious differences once you hear them side by side IMO.
EDIT: And by the way , if you got offended by my answer it was nothing personal .It's just that you are like the fourth person here who wanted me to "break down " and verbally explain some song or genre to them. It's just not something that interests me...I like spending time listening to music and experiencing what it is and means to me ...not explaining it .If others are interested in that more power to them ......I am not .I gave you samples so you could hear it for yourself .
Music is personal I don't want anyone to interpret it for me .
I couldn't hear the differences between the vids you posted as one was disabled. I did it listen to after the fact and I can hear the difference.
Nobolob's description was excellent IMO as I understood his description without any reservations and his videos were great in showing(hearing) the contrast between speed and thrash metal. Again, I could hear the contrasts as they were very apparent especially the harmony vocal choruses and the melodic vocal style of speed compared to the "brash", intense and agressive vocals of thrash.
Listening to Helloween's Keeper Of The Seven Keys, Part 2 last night I couldn't distinguish if it was speed, thrash, or power metal. All three I would say. After listening to Blind Gaurdian and then to Iced Earth I can also hear the difference between European and US power metal.
Maybe my post was grandiose, but that's irrelevent as I thought it was a good description showing the contrasts and examples of the three sub-genres, not only for my benefit but hopefully for others to get a better understanding of it as I read some posts with some people somewhat confused as well.
It wasn't plagiarised as I put the summary in my own words from reading from three different sites. Who doesn't goggle when it comes to doing research on music, bands, genres, etc.(?). If any one says otherwise, IMO is full of shit. For me, with reading about the music, hearing other members feedback, and listening to the music puts it all together for me, most of the time anyway.
I did take your second post personally. Maybe it wasn't intended to be that way. After reading your further posts, I now don't take it personally. I understand everybody has thier own way of listening, describing, and explaining music. I have respect for that, most of the time.