I can try to hold a speed of up to 28 mph on crowded city commutes for as long as possible but without riding like a jerk, my average speed would only be 15 mph or less, or almost half my maximum sustainable speed!

If you're only averaging 11 mph, double that figure, and you could be hitting >20 mph in some segments. >20 mph is not bad at all on a hybrid bike with pannier. I'd even say you're doing fantastic!

So I'd say you're doing perfectly fine I suppose.....But if you say you're only averaging 11 mph in nearly empty flat roads far away from the city, they yes, you have plenty of room to improve on 11 mph speed.

Spend some time on google maps and study the area which you're riding through. It's possible that there are side streets, trails, paths and all kinds of other short cuts which you could connect together to make your commute faster.

It's a habit, it's just what I do.

I've done that.
Throw in some variations.
Also try pedalling standing up.

If it is wrong, I don’t want to be right.

I am experimenting with adding a fairing to my commuter/utility BSO. It does increase speed by about 10% on the flats.

Police Bike Lights and Sirens

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...averaging 10 or 12 MPH, for a city commute, with traffic lights in commuter traffic, is doing pretty good where I live. The only way I could do much better would be to blow a lot of lights.
Is there another route you could take that might be less direct, but have fewer traffic control devices ? For example, I have MUP along the river. It's a little longer distance, but quicker overall to get downtown.

I do a similar 9 mi commute and I take about the same time. Imo, its the nature of urban biking. Lots of stop and go, red light, etc. All there is, is pedal harder or find a more fluid route.

Sure, and quit the clipless pedals. Clipping in and out 10+ times isn't helping either. I recently got some Shimano XT flats. They are great for urban riding with lots of forced stand stills.

Really??
Stop it unless you're trying to imply that the other person said something stupid.

I highly doubt that you or anybody else can hold 28 mph on crowded city streets for as long as you want...

Not even close to what he said.

Sometimes, we get an empty street segment when the light turns green. It's not usually longer than 1 mile until the next traffic stop. 28 mph for only 1 mile is not that hard to do.

Daytime flashing lights and keeping to the center of the roads make it safer. It's unsafe to go that fast on the bike lane.

But my average speed would only be 15 mph. It's not often I get empty segments to hit 28 or even 30 mph. At most, I only get three opportunities to do this on commutes.