I have talked to a number of amateur radio operators in the clubs that I am in and they told me that many people cannot slow down from 25 wpm because they do not recognize code at 5 to 15 wpm.
this is because they learned almost all of the common words, Q-code, and abbreviations as (effectively) single characters - same way as we all read and hear whole words.
I doubt that anyone who normally uses 25 wpm will have difficultes ( other than falling asleep ) with 10 wpm
if you spent 20 years at 35wpm it might sound a bit odd and rather annoying . . .
What they mean is that they're have to work at it like the rest of us rather than just hearing whole words. That is to say . . .
other people will tell you they don't notice the morse anymore and just hear the content as if it was being spoken.
Slow right down and it's like saying:-
"h" "e" "l" "l" "o"
- you now have five characters to copy and remember
then you have to combine them for the word . .
Lots of effort.
If I was you . . .
I would just learn all the code as quickly as you can
Go for speeding up afterwards, which actually will happen without so much effort . . .
Good luck with it anyway.