Is 'low speed' finally proving its merit?

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
slow /slō/
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 = ?

Solution 1:

If you look up this definition of the word speed, you will see that the first meaning attributed to the word is the rate at which someone or something moves or is able to move. So, it is perfectly logical to talk about both slow and fast speed, as the word is neutral in this sense and can be modified with these two adjectives.

I wouldn't call it an error, therefore. Both uses of slow speed and low speed are grammatical.

Solution 2:

Such constructions are exceedingly common in English. You might want to examine your assumption that this is an "obvious error" or "obviously illogical".

• Short stature
• Narrow width
• Short length
• Slow acceleration
• Shallow depth
• Low altitude

Some people consider "slow speed" an oxymoron - but as is the case with most oxymora, the meaning is crystal clear. In language it's successful communication of meaning that matters, not logical correctness. Are you claiming that you don't understand what is being said, that its meaning is not clear? Or are you saying that redundancy shouldn't be used in language? That's incorrect, languages use redundancy all over place. Or do you just not like the construction? That's certainly your prerogative, you can speak that way if you like, but it's not generally correct at least according to other speakers' evidence.