Two words together and hung in front of a noun, like a label on a box: this one is built for speed. The label says what the thing is for, not how fast it is moving right now - a high-speed train standing at the platform is still a high-speed train.
high-speed 的核心畫面可以想成：兩個字用連字號夾在一起，掛在名詞前面，像貼在箱子上的一張標籤，上面寫著「這台是為了快而做的」。標籤說的是用途，不是此刻的速度——高鐵停在月台上，它還是 high-speed train。要講速度本身的時候，英文就換個形狀說 at high speed，連字號通常也就不見了。