Just started 650b commuting.
Right off the bat, I can say that it is a tremendously more comfortable ride than I have had on my aluminum/composite 25mm tire road bike.
Whether from the wheel size, tires, or the fact that it is a lugged steel frame, or a mix of all of these factors.
On the 700x25c roadie, I get jarring pounding up to the back of my neck from bumps I hit that I wasn't out of the saddle for. Lotsa road vibration in hands. But it feels light an fast.
On the 650b road bike (which weighs only 2-3 lbs. more), no neck pounding anymore, and I cruise along like on a cushion of air. It is a very soft an smooth ride.
More control when going onto grass/dirt, too.
Not sure if there is a speed deficit. I am curious and am tracking my average speed on my commutes for comparison.

Yes, it is annoying that I cannot get Gatorskins in 650b size and have to use tires I have never heard of before. I am waiting for someone to make a light, 32mm, bombproof, slick 650b tire.