The Farnsworth method seems to cause quite a lot of problems. There´s two combinations of speed, the character speed and the overall (WPM), which are often quite different.
I read numerous comments from hams who have learned their letters & characters at one specific speed, state they have difficulty reading the characters at a different speed than the one they learned at. This puts them at a disadvantage when they get on air.
Most morse is sent with a matching wpm and character speed. Its also the way most hams send morse. Its the way I send morse by both straight key or using a paddle with a keyer. All the military and commercial operators I know were taught to read morse using a slow overall (WPM) speed and matching character speed with one or two WPM increases when a good acuracy was obtained. This continued until the required ultimate speed and accuracy was reached.
This method has a couple of advantages in that increments in speed are easier to manage and read. In this way as you progress to say, 20wpm or more, it means you can read any morse character sent a slower speeds or much slower speeds, as you are used to hearing and learning the charcters at different speeds. Farnsworth limits you initially to one character speed and with spacing which is often unusual and as I mentioned earlier, not one most operators use.
So if someone sends" QRS", it is not always obvious whether he/she means reduce the character speed, the overall speed, or simply needs a bigger gap between characters.?