That's not casual commuting but serious workout. I could do this when I biked 20 miles a day full speed.

Oh absolutely. I don't think I'm an ass on the road (I stay off sidewalks when I can and give a heads up when I'm passing pedestrians), but I do get into my spandex and treat every ride I do like a serious workout. My commute to work is about 25 miles and try to bike that round trip once a week, and then doing more challenging rides on the weekends.

Impressive. That would win most time trials at my local club. Have you tried racing?

I've been only doing solo and work rides, but I'm trying to get into amateur racing actually and been psyched out getting into a "serious" group of riders, but hearing that is really encouraging :). Of course, this is only flats, but I do try treat almost every ride as a training ride and push myself, and use the weekends to train as well.

How long can you ride at that speed? I'm wondering because I average around 15 on my commuter bike and i'm thinking of getting something faster for my commute.

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.