As I think it is with most Japanese language learners, I find onomatopoeic words to be some of the most fun to learn (and usually easy to memorize, but then occasionally difficult).
I just came across this in the nuance hint in a quiz, that ゆっくり is actually an onomatopoeic word… someone please tell me what that’s the sound of?
Strictly speaking, onomatopoeia only refers to words that describe a sound, but in reality all types of mimetic expressions are sometimes grouped together under this term. Especially in Japanese learning resources. 
Usually even mimetic words are ultimately derived from something that kinda sorta sounds like something, so I was hoping to hear something along those lines, because I couldn’t find anything myself.
This is a pretty fun article, though: Japanese Onomatopoeia: The Guide
I did a bit of googling, and believe this might be the answer you’re looking for
語源 [ゆくら]が、[接中辞] [っ]により強調されたもの。
E.g. ゆっくり is a strengthening of ゆくら,
Another google shows ゆくら is generally something swaying.
So, in my head I’m picturing a tree swaying slowing in the wind - which works quite well for slowly for my mental picture at least.
I love it; thanks for the assist!
May or may not be interesting. But ゆ in general in kun-yomi readings tends to indicate slackness/flexibility/loss of rigidity.
緩む loosen、許す allow、揺れる shake、ゆとり leeway、譲る defer、歪む warp、茹でる soften vegetables through boiling.
yeah this is the good stuff I lurk around for, keep it coming Asher.
I find this angle incredibly interesting and useful for trying to remember things about words, as just wrote learning and SRS are terrible for me for vocab
This is great. Is this kind of information in the *onomatopoeia” deck?
Unfortunately not, as things like this have no basis apart from my own observations. I have always been very vocal about kana having deeper meanings throughout the language as a whole, not just limited to onomatopoeia, but I do not recommend anyone else adopting this assumption without some serious research of their own.
If I am wrong, then I am wrong, but I don’t want to make other people wrong too if my conclusions are incorrect.
The stuff that is in the onomatopoeia deck are the commonly agreed upon ‘feelings’ of kana, with a tiny bit of descriptive liberty.
Understood.
Perhaps there is a link via man’yōgana…
That’s incredibly useful!
The Dictionary of Japanese Grammar series has an appendix on sound symbolism at the beginning of their Basic dictionary, with a very strong recommendation along the lines of “try to understand how this works early in your Japanese learning”.
While they don’t describe ゆ as thoroughly Asher has, they do describe the semi-vowel [y] in general as representing weakness, slowness or softness. Eg:
よいよい (Loss of reflexes/ataxia)、ゆらゆら（Swaying - like waves)、やんわり（Softly）、よぼよぼ（Senile）.