My gut feeling is that this is simply the speed range where the benefits of faster typing during composing quickly fall off. If you're thinking about what to say, you'll write a burst, think, write, think, change a bit, write some more- the limiting factor isn't really WPM.
Transcribing, of course, is another matter, but that is a specialized case.
Will try to feel less satisfied with my writing speed, and see if it will help. Thank you for taking time to share!
For reference, my typing while transcribing rate is in the 100-110 range. My daily-use composition is likely half, simply because I spend most of my time composing in my head.
I will say that the big advantage comes from being completely comfortable touch-typing, without needing to look at the keyboard. Once you've achieved that, the mental load of typing fades in to the background, and you can spend more time considering content instead of the mechanics of creating it.
Are there more effective techniques than just typing? I mean like measured techniques for identifying and improving problematic aspects of typing rather than just going at it repeatedly in the hopes that I'll improve overall?