Ergonomic instruments increase comfort, which increases stamina and lower risks of injuries. Better ergonomy may increase per minute throughput, or decrease it. Rather measure throughput per day or even per week or longer; also factor in the potential consequence of injury leading to using the instrument at a fraction of the average user's speed for a good few months at least.
Most important for me wasn't speed, it was flow. But speed is easy to put a number on.
I would be a lot more cautious about injury if I was doing a lot more sustained writing. But mostly I am doing technical writing and programming, which usually involves a lot of thinking followed by a small sprint of actually typing. If this small sprint doesn't break my flow or train of thought, it's perfect.
Ergonomic instruments increase comfort, which increases stamina and lower risks of injuries. Better ergonomy may increase per minute throughput, or decrease it. Rather measure throughput per day or even per week or longer; also factor in the potential consequence of injury leading to using the instrument at a fraction of the average user's speed for a good few months at least.