I've got a moonlander, and pretty average-to-small hands, I guess. I can comfortably use 2 of the thumb keys on each side. I have to move my hands off of home row to hit the third keys, or the red keys comfortably. I've bound those to functionality that isn't typing related, and that works well for me.

I believe Putnam's work is tied in with the SDL2 transition as well. There have been a number of mentions in interviews that they're doing a lot of profiling and cleaning up Tarn's legacy code to get some good optimizations. I'm expecting some noticeable improvements.