They generally reduce their assist and completely cut out at 28 to 30km/hour, so to actually cycle 30km/hour takes as much effort as it would on a normal bicycle.

Most Dutch cycle at about 25km/hour (whether or not electric) - those cyclists that easily do 30 on normal city bikes are probably the last generation that will switch to e-bikes.

The electric assist on e-Citibikes here in NYC diminishes with speed and cuts out completely at around 15 mph. Above that speed they're just a heavier normal Citibike. I know because I've enjoyed pushing these things to their limits.

There are many different types of e-bikes, and you're probably thinking of what are effective electric mopeds, with big batteries and engines and a throttle control. Those certainly go very fast. They're also not legal in many places, including here in NYC, and aren't necessarily what people mean when they're talking about e-bikes; that's usually referring to pedal-assist.

You could, but most e-bikers choose not to go too fast. Additionally, they often have thicker tires and enourage poor aerodynamic body positions which mean that they decelerate faster.

Most are either 60+ or are leisure cyclists. So they just doodle along at their 25km/h.

Proper road racing enthusiasts can average 35km/h including stops (the joys of a flat country).