I average 12.5-15 on my commute, and max out at 22-23 on flats. I can keep up with most, but not all, ebikes, and am slower than most road bikes.

I probably couldn't keep up with you + ebike if you average 15 on a regular bike.

I wonder how the governor works. Does it just stop assisting at 20, or does regenerative braking automatically kick in?

15 on a commuter sounds about right -- pretty fast actually for most anything I'd call a commmuter bike. I rode a single speed bike for about 8 years before I upgraded to an entry level Cannondale Supersix Evo, which I got late in the season so I ended up with a really, really good deal on a carbon frame road bike. On flats I have no problem maintaining 20-22mph for longer periods of time, but that's after some training. On sprinty bursts on flats I do high 20s, maybe breaking 30mph. On downhills 35mph and I don't feel comfortably pushing past that. In fact I rarely feel comfortably going that fast -- there are just specific stretches of road and hills where I'm not worried about potholes or surprise traffic.

When I started on the road bike doing 15-17mph during rolling-hill after-work rides with minimal intersections and traffic, and I'd say that's comfortably my casual pace that I can keep up for at least 25 miles at a time now, except when I first started I was sweaty and out of breath and now it's an easy commute speed.