It's less a question of average composition (deciding what to write) speed, and more a question of how much I'm keeping in memory. With a slower typing speed, I have to keep more in memory about how I want to finish the thoughts I'm having, and have more difficulty and frustration involved in the process.

In other words, composition isn't a marathon, but a series of sprints. Each sprint is a race to get the thoughts you have out of short term memory and into storage. You'd probaply find your composition speed increase with your typing speed, as you can focus on the next thing to write rather than remembering what you have decided to write.

And I just thought of another way to estimate it - think of the difference in your willingness and ability to write things on a phone or tablet (40 wpm) versus a keyboard (80 wpm) and extrapolate.