"Please learn slowly."
The only thing I have seen so far regards pronunciation were the first 3 units of this course. I do not think they will touch on it again in the future. I have read people complaining a bit about the "voice-reading" on the exercises. So, apparently you cannot trust that 100%. I can't say much about it, though.
I did a quick search to know more and although they each have a corresponding verb and can seem to be used interchangeably, there is a difference in nuance.
A quick explanation is that 천천히 is only an adverb and so is like "slowly" whereas 느리게 is only an adjective and is like "be slow". (I got this from Hinative.)
Here's a link to a detailed and clear explanation about the difference in meaning : http://koreanselfstudyisntlame.blogspot.com/2010/10/vs.html?m=1
천천히 is a conscious effort but 느리다 is out of your control BUT can be used about a person to show the speaker has a negative opinion about that person's speed. Clear examples are given on that web page... :-)