I started out at 20/15, and found the letters all ran together and I was identifying them by the pattern of dits & dahs rather than the sound when I could keep up, which was rare. So I upped the speed to 27wpm character speed but found I still could not keep up at 15 effective. Seeing some posts here that recommended dropping the effective speed to get through the lessons to avoid dropping out, I chose to do that. And it’s worked as far as less 21 or so, but with a few downsides. One is that I cannot hear the letters at 15-20wpm-ish or less - they don’t have a sound anymore, so I don’t recognize them. So I can hear the characters fast but not recognize them for fast copy, and can’t hear the characters slow enough that I can copy. Not so good for getting on the air! When I listen to code on the air the characters run together - it’s like there is no space between them.
So I’ve gone down to 20/15, and really struggled to keep up after the first few groups, but 20/12 seems more manageable so I’m going to stick with that for a while.
Incidentally, I did the ‘Morse Mania’ app on iOS (the Teal and Orange one, not the former Mac app) and learned all the characters & symbols in that at 27wpm. It’s a lot like Morse Machine here, but better on the phone.
I started all this on 8/16/21, if that gives any sense of aptitude.
If you already know all the chars at 27/, ( I guess you didn't start "morse mania" on 20210816 ) then you could try a modified farnsworth where you have 27/27 morse groups but a farnsworth (longer) space between the groups.
You then have to remember the morse which may get you started with "head" decoding.
If you already know all the chars then you are on to the stage of repeat decoding to the point where it is happening automatically without you having to think to decode.
For my money (YMMV) I would be listening to lots of morse . . .
YMMV as ever