ok, I just have to chime in. Just got back from riding 35 miles on my 650B X 2 inch Grand Bois Hetre's - average speed just under 16mph on my all steel touring bike (unloaded), no tailwind, no drafting. I am an average 56 year old bike rider.
HUGE smile on my face. Our roads are all chip seal. I can't imagine riding a 23cm tire, ever. Even the pro's ride at least a 25cm and sometimes a 28cm in road races.
For me, comfort, no pinch flats, no getting bucked off at the slightest bump or pot hole.
I hope you decide on at least a 25cm and hopefully a 28cm.

Feel free to go read the Schwalbe website about speed and tire width and rolling resistance. OH and go have fun