Technically, you should expect the term low speed, not slow speed (which is obviously illogical).
However, it seems the two phrases co-existed as long as one can look back: with low speed fighting a desperate battle to prove its merit.
It is only recently that English users seem to have seriously recognized the difference as this nGram shows.
How did this obvious error survive and even now continue to assert itself? Or is it that from the language point of view, there is an argument that both the phrases are correct, grammatically, especially, semantically?
[Edit-1] Some backgrounder on slow and speed
Adjective: Moving or operating, or designed to do so, only at a low speed.
Adverb: At a slow pace; slowly.
Verb: Reduce one's speed or the speed of a vehicle or process.
If slow = low speed
then slow speed = ?