Speak Slower or Speak Slowlier?

AFAIK the correct grammar for "speak slow" is "speak slowly" (slowly being an adverb). Please correct me if I am mistaken.

But in daily life I have not heard anyone saying "Speak slowlier".

I think I heard folks saying "Speak slower" but I also think it is incorrect.

Which is the best or correct form to convey the message?

Could that be the longish "Speak more slowly?"

Slow can be an adverb as well as an adjective. Not all native speakers know this, and some will tell you it's wrong to say 'Speak slower'. If you don't want to risk upsetting them, you can certainly say 'Speak more slowly'.

The Oxford English Dictionary has two citations for slowlier, including this from the seventeenth century philosopher Thomas Hobbes: 'They marched the slowlier for the rain which had fallen the same night.' It is, however, rare: I have never seen or heard it until now.

Slower can either be an adjective, or an adverb.

"Speak slower" is grammatically correct. "*Slowlier" is not a word.

"*Slowlier" sounds very unnatural to a native English speaker; however, not all know that slower can also be an adverb.

Some incorrectly claim that the adverb "slower" should be avoided. However, it has been in use for about four centuries now and is well established as a real word.