Student or Learner
Please speak (slow/ slowly).
Good morning, Pete.
(1) Here in the United States, my teachers taught me the following:
(a) "slowly" is, of course, the "correct" adverb.
(b) "slow," however, can also be used as an adverb.
(c) Some experts say that sometimes "slow" sounds better.
(d) You are a learner, so you should probably use "slowly."
As you become more proficient in English, you can let your ear decide.
(i) For example, one expert gives this example:
I have Windows 95, but now I'm running out of memory, plus it runs too slow at times.
That expert feels that the "flat" adverb sounds better than the -ly adverb in that sentence.
P . S. Of course, you MUST used -ly before a verb:
He slowly opened the door.
***** HAve a nice day!